DE3014870A1 - METHOD FOR PRODUCING AN EXHAUST GAS ELECTRODE FOR AN EXHAUST GAS OXYGEN SENSOR - Google Patents

Description

" ³ " 3Q14870

The invention relates to an electrochemical exhaust gas oxygen sensor of the solid electrolyte type. In particular, the invention relates to a spray-on method for depositing a platinum exhaust gas electrode on a bowl made of vitrified zirconium oxide for the production of such a sensor.

A typical solid electrolyte exhaust gas sensor for use in automobiles is described in U.S. Patent 3,844,920. He has a sensor element made of zirconium oxide, which is designed as a bowl with sloping side walls. An end of the cup is open and has a flange protruding in the circumferential direction. The other end is closed and forms the active part of the element. The inner surface and the outer surface of the bowl are covered with separate porous elec- electrode coatings made of platinum. The inner electrode is an oxygen source, ζ.B. Air or a metal oxide mixture, exposed to form a reference potential. This electrode was generally made by brushing on a layer of platinum "ink" or platinum slurry on the zirconium oxide cup, and by drying the coating and then heating the coated bowl at a raised temperature manufactured.

The outer electrode, also known as the "exhaust gas" electrode, is exposed to the exhaust gas in order to create a potential that is generated by the Oxygen content of the exhaust gas is determined. The outer electrode can, like the inner electrode, consist of a porous thick layer of platinum. However, it is preferred this outer electrode by vapor deposition, by spraying, by chemical deposition from the vapor phase or by a another thin-film deposition process applied thin-film layer. On the other hand, it has proven difficult to coherently and repeatably create desirable properties, e.g. the porosity and electrical parameters, in the case of thin

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Continuous manufacture of layer electrodes. This gave the yield of electrodes with satisfactory electrode properties very limited and various auxiliary methods have been developed to improve this yield. For example, according to US Pat. No. 3,987,006, the solid electrolyte body is baked out made after electrode deposition in order to form pores in the electrode coating if they are deposited during deposition does not become porous enough. According to US Pat. No. 4,136,000, a chemical and an electrolytic treatment of the sensor element is described and the electrodes are made to improve certain sensor properties. In addition, it is known that Zirconium oxide exhaust gas sensors, especially those sensors that contain a sprayed-on exhaust gas electrode, after a short operating time often change their electrical properties. In general there is even an improvement, e.g. a reduction in the response time after switching. Accordingly, it has already been proposed that such sensors functionally operate in an actual or simulated exhaust gas atmosphere until it stabilizes sufficiently before they are installed in an exhaust system. However, such treatments of course increase that Manufacturing costs. In addition, the yield of sensors with better properties is still due to the quality of the originally deposited electrode film from the start limited.

The method according to the invention makes it possible Spray platinum layers on the zirconium oxide surface in such a way that that the film in the deposited state is porous through and through and has a large surface area that can be reproduced again and again in the deposited state, which results in a greater yield of high quality sensors. It In this way, sensors with a lower response time when switching the display from lean to fat are generated without that further chemical or electrolytic treatments are necessary

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3Q1487Q

are agile. The response times of the changeover process from fat to lean are initially not close either as short as the response times in the direction of lean to fat. However, these response times easily become acceptable low values decrease after even a short actual or simulated aging. This results in a high yield of very quickly responding sensors after a minimal actual aging time of the sensors. Indeed sensors whose exhaust electrodes are made in accordance with this invention are very good for one Aging by a simple furnace treatment, as described in the German patent application filed on the same day P (our reference G 3368) is described. In addition, a considerable number of sensors are required with exhaust gas electrodes produced according to the invention not even an aging treatment for activation and stabilization. The electrical properties in the production state are then more than sufficient and essentially remain stable during initial use as a sensor.

The aim of the invention is thus to provide an improved spray-on method for depositing a platinum exhaust gas electrode on a solid electrolyte body made of zirconium oxide for the production of an electrochemical exhaust gas oxygen sensors to create.

"According to the method according to the invention, platinum is sprayed onto a solid electrolyte body made of vitrified zirconium oxide, which is relatively far away from the platinum target. The target is more than 3.7 cm from the workpiece in a preferred embodiment. The platinum is sprayed on with direct voltage (ie it is sprayed with the help of a rectified current) from the target, and this takes place under a relatively high pressure of 1.333 to 2.666 Pa (= 10 to 20 mTorr), with a spray
finds.

Spray output from 13 to 22 watts / cm target area use

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With the method according to the invention, the outer electrode layer can can be applied to bowls made of vitrified zirconium oxide with a pre-deposited inner platinum layer. These Bowls are 3-5 cm long and are made of zirconium oxide that is partly or fully in its cubic shape Inclusion of about 4-8 mole percent yttrium oxide is stabilized. The outer surfaces of the bowls are bevelled of 3 ° 38 *. In a particularly cited example, the cup has an axial length of 3.66 cm provided with a circumferential shoulder at its wider end and has a diameter of 0.82 cm immediately below the shoulder. The narrower end is closed and rounded and has an outer radius of curvature of 3 mm on. Its diameter near the rounded end is 0.4 cm. It is believed that deposition is important to influence and control at and near the rounded end. This area seems to be the most active To be part of the sensor element. The inner electrode is preferably in the form of a platinum slurry (platinum ink) · burned onto the zirconium oxide surface Platinum thick film formed first. The wells are then cleaned around the outer exhaust gas electrode, which is formed as a thin-film layer to be obtained in the manner according to the invention.

In thin film deposition processes, it is common practice to clean the zirconia surface well prior to depositing the platinum electrode on it, and so is the case with this invention. It is assumed that any recognized high quality cleaning process can be used. In a cleaning process that has been found to be good, ultrasonic degreasing of the zirconium oxide with freon is carried out, then the degreased bowls are heated in air at about 600 C for about an hour , after which the zirconium oxide bowls are placed on a stainless steel tray with which they are in a sub

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pressure chamber can be used. The vacuum chamber will then up to a residual pressure of 13.33 Pa (= 100 inTorr)

or
pumped out and at / below this negative pressure, the zirconium oxide bowls are reheated to a temperature of about 200 C and held for 10 to 15 minutes. If the zirconia cups are still warm, the vacuum chamber will be refilled with dry nitrogen to a pressure of more than 133.3 Pa (1 Torr) in about one second. The chamber is immediately pumped out again to a pressure of less than 266.6 χ 10 Pa (= 2 χ 10 Torr). A stream of argon at about 75 to 100 cm / min is then allowed to flow through the vacuum chamber while the pump continues to run. The argon flow and the pumping out are maintained and the vacuum delivery is adjusted so that the pressure in the vacuum chamber is set at 1.333 to 2.666 Pa (= 10-20 mTorr). This argon pressure is maintained dynamically in the same way during the spraying process. This means that the atmosphere in the vacuum chamber is continuously refreshed or circulated during the spraying process. With the method according to the invention it is possible to simultaneously spray electrode layers onto large groups of zirconium oxide cups, for example more than 200 cups at the same time. The tray with the wells is placed directly on a horizontal, flat steel pallet carrier. The cup axes are arranged parallel to one another and aligned vertically. The open ends of the bowls rest on the tray. The pallet carrier is placed about 0.5 to 1.5 cm above the anode. It was not tested whether a separation from the anode is necessary at all. The closed ends of the wells face upwardly toward a planar platinum target surface of a cathode which is approximately 3.8 cm above the closed ends of the wells. The spray target is a platinum layer that is applied to and connected to a support plate made of copper. The target is assembled with a cathode, which has a water cooling device and a magnet assembly

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301487Q

Contains order. An argon pressure of 1.333 to 2.666 Pa (= 10-20 inTorr) is, as already mentioned, maintained dynamically in the spray chamber. Between the target and a DC voltage of approximately 500 to 800 volts is applied to the anode. The spray performance will be like this set that a power between target and

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Anode of approximately 13-22 W / cm target area. No special precautions are taken on the tray, pallet support or anode to heat or cool the bowls during the spraying process.

The spraying is continued under the conditions described, the argon pressure of 1.333-2.666 Pa (10-20 mTorr) is maintained until a weight of 10 mg platinum has been deposited on each element. Through this a porous crystalline platinum layer 1.0-1.5 µm thick is deposited at the rounded end, a layer 0.65-1.0 µm thick at a distance of about 0.5 cm from this end, and with a thickness of 0.3-0.75 µm at a distance of about 2 cm from the rounded end. Layer thicknesses in the range mentioned are achieved by spraying on achieved with a duration of 3-5 min under the stated conditions. It is believed that the porosity of the electrode and possibly the surface area as well as the resulting electrical properties of the sensor are a function of the Electrode thickness are. However, it is also the temperature at which the ceramic settles itself during deposition heats up, important. With the method according to the invention there is an apparent electrode area size that is at least has twice the value of the geometric surface size.

With "apparent surface" the surface size is in meant the film-covered area of the element as determined by conventional gas adsorption methods.

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"Geometric surface size" is from a drawing of the element meant certain surface area. In In some or many exemplary embodiments of the method according to the invention, there is a four-fold increase in the apparent Surface size in relation to the underlying zirconium oxide surface, if it is not particularly rough. The adhesion of the sprayed-on layer to the underlying zirconium oxide surface can be improved by Bake out the element provided with electrodes in air at 800 C for one hour. Such a bakeout process appears to reduce the apparent surface area considerably and can create several large pores in the film in one Open thickness from 0.5 to 5 µm. The number and size of these openings seems to depend on the time and temperature of the Bakeout and depends on the thickness of the film. The reduction of the apparent surface area during the heating process does not appear to be unduly detrimental to the sensor properties. On the other hand it seems It is important that the film in the deposited state has a high porosity and surface area. If this isn't the If so, the effectiveness of the sensor is adversely affected by the heating process.

The electrode film is then preferably with a porous Ceramic coating covered in all parts, except where electrical connections have to be made. the porous ceramic coating can be catalytic or non-catalytic as required without incurring substantial initial operational impact on the resulting sensor. For example, a gamma aluminum oxide coating prepared and applied as described in US Pat. No. 4,116,883. Preferably however, it becomes a magnesium aluminate spinel coating by flame spraying applied to the electrode film after it is heat-treated to increase the electrode adhesion

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became. It is recognized that flame spray application of the porous overlayer appears to be considerable can cause physical change in the electrode film. It still appears that the important functional properties of the electrode film essentially unchanged by the flame spraying stay. As a result, the properties of the electrode film in the just deposited state are more fundamental Importance.

As previously mentioned, the platinum target or cathode is attached at a distance of at least 3.5-4.0 cm, preferably about 3.8 cm above the closed ends of the zirconium oxide cups and about 7.6 cm above the anode. This greater than normal spacing is believed to result in improved electrode porosity and process controllability. The mentioned distance seems to be important. If there is a distance of "less than 3.0 cm between the closed ends of the zirconia cups and the target, films with lower porosity appear to result. On the other hand, a distance of more than 4.5 cm appears unnecessary and harmful. Higher performance values are obtained and higher argon pressures are required to achieve a satisfactory plating rate. Platinum deposition is also likely to occur in undesired areas of the device. In addition, the properties of the electrode film become less reproducible. The nature of the platinum target is no more important to this invention than to other sprayings of platinum. the target may be commercially available and should preferably have a high purity platinum surface. However, it is recognized that it may be important in certain circumstances, that small amounts of other metals in the platinum target are present, for example, a few percent

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up to about 5% palladium and / or rhodium. However, because of the cost, a platinum target is preferred in which the platinum or a platinum alloy is the only surface coating.

A power of at least 13 W / cm target area is necessary. Lower performance values apparently result in deposition rates that are too low, so that layers with do not form significant porosity and insufficient surface area result. Conversely, power values of more than 22 W / cm target area result in too great a load for the System components. The higher performance values can also cause platinum deposition in undesirable areas bring the vacuum chamber with it. In any case, the sensor effectiveness is less reproducible if performance

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genes of less than 13 W / cm target area are present. Pressure values of 1.333-2.666 Pa (= 10-20 mTorr) are also preferred, even if this higher pressure range requires higher performance values. At pressure values of less than 1.333 Pa (= less than 10 mTorr) the coating appears less porous, probably because the deposition rate is too low. The separation rate also decreases again at pressures above 2.666 Pa (= 20 mTorr). For this purpose, deposition can also begin to occur in undesired areas of the vacuum chamber.

The spray-on method according to the invention results in electrodes which, in the just deposited state, have improved properties. For example, response times for switching from lean to fat of less than 600 ms can be achieved reproducibly under commercial production conditions. Even in the just separated state, response times of less than 1200 ms for switching from fat to lean are repeatedly generated. After a relatively short aging period or a relatively short operating time of around one hour in exhaust gas, the response time for a rich to lean switchover drops consistently to values below 600 ms. The Um-

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Switching takes place in the range of about 1 5 parts air to one part fuel, i.e. on the lean side and within 0.5 values of air / fuel ratio. Thus, if the sensor does not respond quickly enough in the state it has just been manufactured, it can be repeated and can be safely aged under operating conditions, resulting in a quickly reacting sensor. aside from that the sensor properties that are achieved with electrodes produced according to the invention are more reproducible. There are higher yields of fast-reacting sensors due to functional aging.

In addition, it was found that simple furnace treatment in a nitrogen atmosphere caused artificial aging allowed by sensors with electrodes produced according to the invention. The oven treatment is as already mentioned, described in the simultaneously filed patent application P (our reference number: G 3368). It is talk of DC magnetron spraying; it can however, the basic process steps also apply to normal DC voltage spraying and HF spraying can be applied. The electrode according to the invention can be more durable if a platinum cermet strip is first applied the outer surface of the zirconium oxide body is applied and baked will. If a narrow strip is used, it need not be porous and can be made from a commercially available Platinum slurry (platinum ink) are manufactured, the adheres well to the zirconium oxide after baking which also adheres well to the platinum that has been sprayed on and deposited over it.

The method was described using an exemplary embodiment, in which an argon atmosphere is used as the environment for the spraying process. As in the simultaneously filed Patent application P (our ref .: G 3367)

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disclosed, however, an atmosphere with a total pressure of 1.333-2.666 Pa (= 10-20 mTorr) can also be used which consists primarily of nitrogen and / or oxygen. In the said patent application it is described that such an atmosphere gives considerably better results than an argon atmosphere, when platinum is sprayed on according to the invention. Preferably an atmosphere of 67-75% nitrogen, Remainder argon, used.

Recent studies seem to indicate that the inventive method also in the production of Electrodes can be used, which consist essentially of palladium. However, there seems to be a predominantly one Nitrogen and / or oxygen formed spray atmosphere no improved results or even worse results to provide.

This results in a method for spraying a platinum exhaust gas electrode onto a cup made of vitrified zirconium oxide for the production of an electrochemical exhaust gas oxygen sensor. Porous platinum films with a high specific surface area are deposited, which show more uniform properties. A platinum target is held at a distance of 3.0-4.5 cm from the bowl and more than 6 cm from the spray anode. A pressure of 1.333-2.666 Pa (= 10-20 mTorr) is used during the spraying process and a DC ^{power output of 13-22 W / cm 2} target area.

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Claims (3)

Patent claims: 1. Method of spraying a platinum exhaust electrode onto a Solid electrolyte body made of vitrified zirconium oxide for the production of an electrochemical exhaust gas oxygen sensor, characterized in that a platinum target arranged at least 3.0 cm from the body during spraying that the platinum is used at a pressure of 1.33322 up to 2,66644 Pa (~ 10-20 mTorr) is sprayed on and that 2 a spray power of 13 to 22 W / cm target area is used so that the platinum electrode is deposited as a porous film having an apparent area of at least twice the geometric area of the underlying zirconium oxide surface owns. 0300A5 / 07U 2. The method according to claim 1 for spraying a platinum exhaust gas electrode on a solid electrolyte body made of vitrified zirconium oxide in the form of a cup for production an electrochemical exhaust gas oxygen sensor, thereby characterized in that the platinum target is a generally planar, perpendicular to the axis of the zirconium oxide upright platinum target 3.0-4.5 cm from one closed end of the cup was used becomes, whereby the platinum electrode is deposited consistently and repeatably porous and an apparent Has surface size in the deposited state, which corresponds to at least 4 times the geometric surface size. 3. The method according to claim 2 for spraying a platinum exhaust gas electrode on a bowl made of vitrified zirconium oxide for the production of an electrochemical exhaust gas oxygen sensors, characterized in that a well with a length of 3 to 5 cm is used that a generally planar platinum target is used which is oriented perpendicular to the well and spaced from 3.5 to 4.0 cm from the closed end of the cup and that the platinum is made with a DC magnetron process long enough to produce an electrode thickness greater than 0.65 µm at the end and 0.3-0.5 µm at the Create cup sidewalls, with the deposited platinum electrode an apparent surface area in the as-deposited state of at least four times the geometrical surface area owns. 0 30045/0 7U.