JPS6255621B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an electrochemical type exhaust gas oxygen sensor with a solid electrolyte.
(sensor) related. For more information, please refer to the vitrified zirconia thimble for this type of sensor.
platinum exhaust electrode (zirconia thimble)
The present invention relates to a sputtering process for depositing an exhaust electrode.

A typical automotive solid electrolyte exhaust gas oxygen sensor
gas oxygen sensor) is Burgett
No. 3,844,920 to et al. It is a tapered thimble.
zirconia sensing element
element). One end is open and includes a circumferential flange. The other end is closed and forms the most active part of the device. The interior and exterior of the thimble are provided with separate platinum porous electrode coatings.
To determine the reference potential, the internal electrodes are exposed to an oxygen source such as air or mixed metal oxides. The electrodes were generally made by coating a zirconia thimble with platinum ink, allowing the coating to dry, and then firing the coated thimble at a high temperature.

Also, an external electrode, named exhaust electrode, is exposed to the exhaust gas to determine a potential determined by the exhaust gas oxygen concentration. The outer electrode, like the inner electrode, is a porous thick membrane of platinum. However, this external electrode is
terring), chemical vapor deposition
Preferably, it is a thin film produced by deposition or other thin film deposition techniques. On the other hand,
It has been difficult to consistently reproduce desired properties such as porosity and electrical parameters in thin film electrodes.
As a result, the yield of electrodes with satisfactory electrode properties has been limited, and various auxiliary methods have been developed to improve the yield. For example, U.S. Pat. No. 3,978,006 to Topp et al. discloses that by heating a solid electrolyte body after electrode deposition, pores are formed in the electrode coating if the deposited electrode coating is not porous. Discloses that. US Pat. No. 4,136,000 to Davis et al. discloses chemically and electrolytically treating sensing elements and electrodes to improve sensor properties. Furthermore, it is known that zirconia exhaust gas sensors, particularly those having sputtered exhaust electrodes, appear to change their electrical characteristics after a short period of time during operation. Generally, the improvement is such as a reduction in switching response time. It has therefore been proposed to operate a sensor of this type functionally in an actual or similar exhaust gas flow until it is sufficiently stable before installing it in an actual working system. Of course, such processing adds to manufacturing costs. Moreover, the yield of high performance sensors is inherently limited by the quality of the original deposited electrode film. The method of the present invention sputters a platinum film onto a zirconia surface by a method in which the film is porous and has a high surface area when deposited, contributing to a high yield of high quality sensors. make it possible to low lean-to-rich switching response time
response time) is produced without subsequent chemical and electrolyte treatment. rich-to-lean switching response time
is initially not as low as the lean-to-rich switching response time. However, after maturation in real or simulated conditions for a short period of time, rich-to-lean switching response times generally readily decrease to acceptable low levels. Therefore, high yields of extremely fast-reacting sensors are obtained with only minimal practical aging of the sensors. In fact, the co-pending application of this application is U.S. Patent Application No.
As disclosed and claimed in Publication No. 141664, sensors with exhaust electrodes according to the invention are amenable to aging by simple furnace treatment. Moreover, the sizeable proportion of the sensor with exhaust electrodes according to the invention does not require any aging treatment for activation or stabilization.
These electrical properties as generated are more than sufficient and essentially stable even during the initial use of the sensor.

It is an object of the present invention to provide an improved sputtering method for depositing platinum exhaust electrodes on zirconia solid electrolyte bodies for electrochemical type exhaust gas oxygen sensors.

The invention involves sputtering platinum onto a solid electrolyte body of fused zirconia widely spaced from a platinum target.
In a preferred embodiment, the target is at least 3.7 cm away from the workpiece. Platinum is 10 to 20
Direct current sputtering (direct current sputtering) is performed under relatively high pressures of milliTorr (1.33322 to 2.66644 Pascals) and with a sputtering power of 13 to 22 Watts/cm ² per target area.

A fused zirconia thimble with a predeposited internal platinum electrode can have an external electrode added thereto according to the present invention. These thimbles are approximately 3 to 5 cm long and are made of zirconia that is partially or fully stabilized in the cubic crystalline system with approximately 4 to 8 mole percent yttria. ing.
Each thimble has a taper of approximately 3 degrees and 38 minutes on its outer surface. In a particular embodiment, the thimble has an axial length of 3.66 cm. Its wide end has a circumferential shoulder and the diameter just below the shoulder is 0.82 cm. The narrow end is closed and rounded, and the radius of curvature of the outer sphere is 3 mm. The diameter adjacent to the rounded end is 0.4 cm. It is believed to be important to control precipitation on and near the rounded tip. This shows that this is the most important part of the sensing element. The internal electrodes are first formed on the thimble, preferably as platinum ink, by firing a thick film onto the zirconia surface. The thimble is then cleaned to receive the thin film external exhaust electrode according to the present invention.

Since any thin film deposition method can be used,
Prior to depositing a platinum electrode according to the present invention, the zirconia surface must be thoroughly cleaned. It is expected that any conventional and high quality cleaning method will be used. One cleaning method that can be used is
It involves ultrasonic degreasing of zirconia with Freon and heating the degreased thimble at about 600° C. in air for about 1 hour. The zirconia thimble is then placed on a stainless steel pallet and the pallet is placed in a vacuum chamber. The vacuum chamber is then lowered to a pressure of approximately 100 millitorr (13.3322 palcal) using a vacuum pump. At or below this low pressure, the zirconia thimble is heated again to a temperature of about 200°C for about 10 to 20 minutes.
Heat for 15 minutes. While the zirconia thimble is still warm, the vacuum chamber is backfilled with dry nitrogen to a pressure of greater than 1 Torr (133.322 Parkal) for about 1 second. The vacuum chamber is then immediately pumped down to a pressure below 2 x 10 ^-6 Torr (266.644 x 10 ^-6 Pascals). Then, while the pump continues to run, it is pumped through the vacuum chamber at 75 or
Begin flowing 100 c.c. of argon. Continue the argon flow and pump down and adjust the vacuum valve to bring the pressure in the vacuum chamber to 10 to 20 millitorr (1.33322 to 2.66644 parkals). Similarly, this argon pressure is maintained dynamically during the sputtering process. Thus, the atmosphere in the vacuum chamber is kept continuously fresh during sputtering. In the method of the present invention, it is also possible to sputter electrode films simultaneously onto large groups, such as groups of 200 or more such zircon thimbles. The thimble pallet is placed directly onto a horizontal, flat steel pallet carrier.
The axes of the thimble are parallel and perpendicular to each other. Place the open end on the pallet. Place the pallet carrier 0.5 to 1.5 cm above the anode. However, there is no question as to whether some kind of isolation is necessary. The sealed end of the thimble faces upward toward the flat platinum target face of the cathode, and the target face is placed on the sealed end of the thimble.
They are spaced 3.8cm apart. The sputtering target has a supporting copper backing plate.
A platinum sheet bonded to a copper backing plate. The target is assembled with a water cooler and a cathode containing a magnet array. An argon pressure of 10 to 20 millitorr (1.33322 to 2.66644 Pascal) is dynamically maintained in the sputtering chamber. A DC voltage of about 500 to 800 volts is applied between the target and the anode. The supplied sputtering power is applied between the target and the anode.
Approximately 13 to 22 watts per target area
Adjust to provide DC power of ^cm2 . No special equipment is used on the pallet, pallet carrier or anode to heat or cool the thimble during sputtering.

Sputtering is continued under the above conditions until 10 mg of platinum has been deposited on each element while maintaining the argon pressure between 10 and 20 mTorr (1.33322 to 2.66644 Pascals). This is 1.0 to 1.5 micrometers thick on the rounded end, 0.65 to 1.0 micrometers about 0.5 cm from the round tip, and about 2
A porous crystalline platinum deposit with a thickness of 0.3 to 0.75 micrometers is created at cm. Coatings of such thickness can be obtained by sputtering for 3 to 5 minutes under the above conditions. It is believed that the porosity and possibly surface area of the electrode and the resulting electrical properties of the sensor are a function of the electrode thickness. Also important is the temperature due to self-heating of the ceramic during precipitation. The technique of the present invention must provide an apparent electrode surface with an area at least twice its geometric area. Apparent surface area refers to the surface area of the coated portion of the device as measured by conventional gas absorption methods. Geometric surface area means the surface area measured from the diagram of the device. In many instances, the method of the present invention provides a four-fold increase in the apparent surface area deposited on the underlying zirconia surface, provided the surface area measurements of the zirconia surface are not particularly rough. Adhesion of the sputtered film to its underlying zirconia surface can be increased by heating the electrode element to 800° C. for about 1 hour in air. Such heating results in a significant reduction in the apparent surface area and can create several large holes, 0.5 to 5 micrometers wide, on the membrane. The number and size of these openings depends on the time and temperature of heating and the thickness of the film. This reduction in apparent surface area during heating did not appear to be significantly detrimental to sensor performance.
On the other hand, it seems important that the membrane has high porosity and high surface area as a precipitate. Otherwise, sensor performance will be adversely affected by this heating.

The electrode membrane is then preferably provided with a porous ceramic coating on all other parts except where electrical contact is to be made. If the resulting sensor has no significant initial operation effects,
Porous ceramic coatings can be catalytic or non-catalytic. For example, gamma alumina manufactured and applied as disclosed in U.S. Pat. No. 4,116,883 (Rodes).
alumina) paint can be used. However, it is advantageous to flame spray a magnesium aluminate spinel onto the electrode membrane after heat treatment to increase electrode adhesion. It has been observed that the application of a porous topcoat by flame spraying significantly alters the appearance and physical appearance of the electrode membrane. However, the essential functional properties of the electrode membrane are
It is clear that it remains essentially unchanged even after flame spraying. Therefore, the characteristics of the electrode film as a precipitate are fundamentally important.

As mentioned above, the platinum target or cathode is spaced at least 3.5 to 4.0 cm above the closed end of the zirconia thimble, preferably about 3.8 cm, and about 7.6 cm above the anode. It is believed that this spacing, which is larger than normal spacing, provides improved electrode porosity and better process controllability. The above interval appears to be critical. If the distance between the closed end of the zirconia and the target is closer than 3.0 cm, the porosity of the resulting membrane is obviously reduced. Spacing greater than 4.5 cm is unnecessary and should be opposed. High power settings and high argon pressures are required to obtain satisfactory coating speeds. It appears that platinum deposits tend to occur in unnecessary parts of the device. Furthermore, the reproducibility of the characteristics of the electrode film seems to be low. The nature of the platinum target is less critical for the present invention than for other platinum sputterings. Targets are available from any commercial manufacturer and preferably provide a high purity platinum surface. However, it is recognized that in some instances it may be desirable to include small amounts of other metals within the pure platinum platinum target, such as several percent, up to 5 percent, of palladium and/or rhodium. For reasons of cost, it is preferable that the platinum target has only a surface coating of platinum or a platinum alloy.

A power setting of at least 13 watts/cm ² is required on the target surface. Low power produces too slow a deposition rate to obtain significant porosity and surface area. Conversely, power settings of 22 watts/cm ² or higher seem to be harsh on the system components. Also, high power settings result in platinum deposition in unwanted areas within the vacuum chamber. In any case, the reproducibility of sensor performance is poor below 13 watts/cm ² on the target surface. A pressure of 10 to 20 millitorr (1.33322 to 2.66644 Pascal) is preferred. This high voltage range is preferred even if high power settings are used. At pressures below 10 mTorr (1.33322 Pascals), the coating appears to be less porous because the polyanalyte extraction rate is too slow. The precipitation rate also decreases at pressures above 20 mTorr (2.66644 Pascals). Also, precipitation begins to occur in unnecessary areas within the vacuum chamber.

The sputtering technique of the present invention provides an improved electrode as a deposit. For example, lean-to-rich switching response times of less than 600 milliseconds can be consistently produced under industrial manufacturing conditions. Manufactured as a precipitate with a constant rich-to-lean switching response time of less than 1200 milliseconds. However, after relatively short periods of operation in exhaust gases, on the order of one hour, the rich-to-lean switching response time consistently drops below 600 milliseconds. Controllability is approximately
For an air/fuel ratio of 15 parts air to 1 part fuel, or about 0.5 on the lean side. Therefore, even if the sensor is not fast acting as a molded article, certain operational aging provides a fast acting sensor. Furthermore, the sensor characteristics obtained using the electrodes manufactured according to the present invention are highly reproducible. High yields of fast acting sensors are obtained by operational ripening.

Furthermore, it has been discovered that sensors with electrodes according to the invention can be artificially aged by a simple furnace treatment in nitrogen. Furnace processing is described and claimed in the aforementioned combined application, US Patent Application Serial No. 030,747. DC magnetron sputtering
is disclosed here. However, the principles of the present invention are also applicable to normal DC sputtering and high frequency sputtering. Furthermore, the electrodes of the present invention are more durable if a platinum cermet stripe is first applied to the outer surface of the zirconia and fired. If narrow, the stripes need not be porous and any commercially available platinum ink can be used that adheres well to the zirconia when fired and to which the sputtered platinum deposited adheres.

It should also be noted that in the specific embodiments of the invention described above, the platinum was sputtered under an argon atmosphere. However, the present invention is co-pending US Patent Application No. 030748 (Japanese Patent Application Publication No.
55-141666), an atmosphere of 10 to 20 mTorr (1.33322 to 2.66644 Pascal) mainly of nitrogen and/or oxygen can also be used. This co-pending US patent application discloses that such an atmosphere provides significantly improved results over argon during platinum sputtering of the present invention. 67
An atmosphere of between 75% nitrogen and the balance argon is preferred.

Recent tests have shown that the method of the invention is also effective for producing electrodes consisting essentially of palladium. However, primarily nitrogen and/or oxygen sputtering atmospheres are not suitable or even desirable for sputtering this type of electrode.

Claims (1)

[Claims] 1. In a method of sputtering a platinum exhaust gas electrode on a solid electrolyte body made of fused zirconia for an electrochemical type exhaust gas oxygen sensor, a platinum target that is at least 3.0 cm away from the main body is used for sputtering. , sputtering platinum at a pressure of 10 to 20 millitorr (1.33322 to 2.66644 palcal) and using a sputtering power of 13 to 22 watts/cm ² for the target area, the platinum electrode was placed on the zirconia surface on which the membrane rests. A sputtering process characterized in that it is deposited as a porous film having an apparent surface area at least twice the geometric area. 2. In a method of sputtering a platinum exhaust gas electrode onto a thimble-type fused zirconia solid electrolyte body for an electrochemical type exhaust gas oxygen sensor, the platinum target is a substantially flat platinum electrode oriented perpendicular to the axis of the zirconia thimble. The target is located 3.0 to 4.5 cm away from the closed end of the thimble, so that the platinum electrode remains unchanged as a precipitate, is reproducibly porous, and as a precipitate, its geometric surface area at least 4
The sputtering method according to claim 1, wherein the sputtering method has double the apparent surface area. 3. A method of sputtering a platinum exhaust gas electrode onto a fused zirconia thimble for an electrochemical type exhaust gas oxygen sensor, in which the thimble is 3 to 5 cm long and the approximately flat platinum target is oriented perpendicular to the thimble. 3.5 to 4.0 cm away from the sealed end on the thimble, with a thickness of 0.65 micrometers or more on said end, and 0.3 to 0.5 micrometers on the side wall of the thimble.
Claims characterized in that direct current magnetron sputtering of platinum for a time sufficient to produce a platinum electrode having an apparent surface area of micrometers and having as a deposit at least four times its geometric area. The sputtering method according to item 2.