DE2934637A1 - MANUFACTURING METHOD FOR AN ACTIVATED OXYGEN PROBE - Google Patents

Description

The invention relates to a manufacturing method for an activated Oxygen sensor.

As explained in our earlier application, oxygen sensors are used with dry-filled oxygen sensor elements for measuring the oxygen content of a motor vehicle exhaust gas for the purpose of regulating the efficiency of the engine by monitoring or controlling the mixing ratio from air to fuel. These generally thimble-shaped sensor elements have on the inside of the thimble an inner conductive catalyst electrode and an outer conductive catalyst electrode on the outside of the thimble, which are interconnected to form a monitoring and actuation device to adjust the air-fuel mixture ratio.

According to our earlier registration, the inner electrode was provided with a inorganic acid or an acid salt brought into contact, which caused a chemical treatment of the inner electrode and a increased the positive output voltage of the sensing element and also decreased the internal resistance of the dry fill sensing element; both features were beneficial to the operation of the probe. As further explained in this application, were the properties mentioned are further improved by the fact that Chemically activated sensor elements were also exposed to a direct current, with the outer catalyst electrode at high Temperature and in the presence of a reducing gas acted as the cathode, and also the response time for switching the Measured values for a rich to a lean gas mixture shortened.

030013/0662

When using the combined chemical and current activation treatment however, there was a need for the presence of a reducing gas during current activation on the outer electrode as well as the need for a recovery period in which the probe was held at the high temperature to create a stable state in the dry filling.

According to the latest findings, a non-oxidizing gas only needs to be present when the current activation of the sensor is activated the chemical activation is combined, the current activation being carried out by applying a direct current to the sensor is, and the outer electrode acts as an anode; In addition, there is no longer any need for a recovery period if the electricity is only available for short periods Period is created.

The invention relates to a manufacturing method for an activated Oxygen sensor element, which gives an increased output voltage with rich mixing ratios, a shorter changeover or Switching response time and a reduced internal resistance, which also has a dry filling such as zirconium dioxide with a conductive inner catalyst electrode on the inner surface and a conductive outer catalyst electrode on the outer surface, thereby characterized in that the conductive inner catalyst electrode is in contact with an inorganic acid or an acidic salt is brought and that a direct current is applied to the sensor element, the outer electrode working as an anode, further in that the current is applied as long as the outer electrode is in the presence of a non-oxidizing atmosphere and is at a temperature of over 450 C, and finally that

030013/0662

5 mA / cm of the flat surface of the conductive outer catalyst electrode.

by that the density of the applied current is at least 5 mA / cm of the

Gas sensor elements manufactured according to the invention for improvement their properties are generally in the form of a closed tubular thimble-like body made from a solid electrolyte or a dry fill such as stabilized Zirconia is formed. This general form of dry fill as well as the solid electrolyte used are general known. The thimble shape of the sensor element with a Approach at its open end is shown in U.S. Patent 3,978,006 and others Publications shown that also deal with the various dry fillers that are used to form such Serve sensor elements. The preferred composition for forming the dry filling is a mixture of zirconium dioxide and stabilizing substances such as quick lime or yttrium oxide.

An inner electrode is made on the inner surface of the dry filling applied to a conductive catalyst material, for example by Coat the surface with a platinum paste with or without glass frit or another high-temperature-resistant adhesive. This pasty coating generally covers the inner surface of the closed connection side and extends to the approach of the dry filling or the electrolyte body. This mixture is then kept at a temperature of 600-10OO C or higher for a sufficient period of time Fired to convert the platinum paste into an electrically conductive inner electrode.

Although a glass frit or other glue is an excellent one

030013/0662

Adhesion of the catalyst electrode to the inner surface of the dry filling offers, it causes an increase in the electrical internal resistance of the sensor, which increases the positive output voltage of the Sensor is degraded when its outer surface is exposed to an enriched atmosphere or a rich mixture, and also a negative output voltage is generated when its External surface of a lean atmosphere or a lean mixture ratio is exposed.

After an earlier registration by the same applicant, the senior Catalyst electrode on the inner surface of the dry filling is subjected to a chemical activation treatment to increase the output voltage to increase and decrease the internal resistance of the sensor. Treatment of the conductive inner catalyst electrode takes place by touching its surface with a solution of an inorganic acid or an acidic salt. Solutions of an inorganic Acids such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid and hydrochloroplatinic acid are preferred, however, acid salts such as ammonia, hydroxylamine chlorohydrate, platinum almiac or the like. can also be used.

When treating the conductive catalyst electrode with an acid or acid salt solution, the electrode can with the solution contacted for a period of time before removing and rinsing the solution; those in contact with the solution However, the electrode can also be heated to evaporate the solvent from the solution and then further to high temperatures be heated in the range of about 1200 ° C.

In addition to the chemical activation of the in-

030013 / nfi62

In the inner electrode of the sensor element, the outer electrode is subjected to activation by means of a current treatment.

Both conductive catalyst electrodes can be platinum or catalysts from the platinum family such as palladium, rhodium or alloys thereof, the preferred material being platinum.

In the current treatment according to the invention for activation, a Direct current is applied to the sensor, the conductive catalyst outer electrode acts as an anode in the presence of a non-oxidizing gas and at a high temperature. This current activation will be more detailed in the application: Process for Producing a Solid Electrolyte Oxygen Gas Sensing Element a dry fill oxygen sensing element) which is incorporated herein by reference. According to this co-pending Patent application is the outer surface of the dry filling body on which the outer conductive catalyst electrode is applied is exposed to a non-oxidizing atmosphere, and while the outer surface is above 450 ° C, a DC current is applied to the sensing element, with the outer elec-

trode acts as an anode, the current density of which is at least 5 itiA / cm of the

flat surface of the conductive outer catalyst electrode.

The non-oxidizing atmosphere that the outer electrode during exposed to current activation may be a reducing, neutral or inactive atmosphere, provided that it is non-oxidizing. Carbon monoxide, hydrogen or rich exhaust gas mixtures are examples of reducing atmospheres, nitrogen is the preferred neutral gas and argon is an example of

030013/0662

~ ⁹ "2934B3?

an inactive or noble gas. Of course, mixtures of a reducing gas and a neutral or a noble gas can also be used and a small amount of water vapor may also be present in the gas mixture.

Before the direct current is applied, the outer surface is heated to a temperature of around 450 ° C, which, however, depends on the dry filling used and other processing conditions can be up to 1100 C. A preferred temperature range from 600-9OO ° C represents an economical and effective Temperature range for power activation.

The direct current is applied to the sensor element so that the conductive Outer catalyst electrode at high temperature and in the presence of a non-oxidizing gas as the anode and the conductive one Inner catalyst electrode serves as a cathode. A DC voltage source is therefore connected in such a way that the outer electrode is connected to the positive and the inner electrode is connected to the negative terminal of the voltage source.

The current charge for current activation has a current density

of at least 5 mA / cm of the flat surface of the conductive outer catalyst electrode. The term "current density", as it applies in this description, results from the division of the current in ' Milliamps through the flat surface in cm of the conductive outer catalyst electrode on the outer surface of the dry filling, while the expression "flat surface of the outer electrode" is used to denote serves the surface that would be present if the conductive catalyst electrode would be a smooth surface without pores. The preferred one

030013/0662

The current density range is between 20 and 150 mA / cm of the surface the conductive outer catalyst electrode. Current densities below

5 mA / cm gives no noticeable effects, although higher current densities can be used, and current densities well in excess of the preferred range can be impacted to break the E learn η te s to guide.

If the direct current is applied for a period of only about 2 seconds as described above, the desired values are obtained Properties, however, a period between 6 see and about 10 minutes is preferred for the direct current to be applied. However, the longer acting times of the current may require a recovery period for the dry fill to stabilize. These Recovery period is obtained by leaving the outer surface of the sensing element in non-oxidizing for a period of time after the power is turned off Gas and at elevated temperature remains.

The following examples serve to further illustrate the invention. In these examples, thimbles were tested as sensor elements in order to determine their performance based on the output voltage with a rich and lean mixture ratio, the switchover or changeover behavior to a gas change and their internal resistance, the tests being carried out in such a way that the thimbles were inserted in protective housings, leads were connected to the inner and outer electrodes so as to give probes. The measurements were carried out at 35O ° C and at 800 ⁰ C, the measurement at 800 C was carried out first.

The performance tests for sensors were carried out by

030013/0662

the sensors inserted into a cylindrical metal tube and they then exposed in the tube to an oxidizing and a reducing gas atmosphere with the aid of a gas burner, which such atmospheres can generate by setting. The measuring sensors arranged in the desired positions in the pipe were heated to the measuring temperature, and the output voltage was measured with the aid of a voltmeter. The output was also fed to an oscilloscope, to measure the reaction rate of the sensor when the burner flame changes from a rich to a lean one and from a lean one is switched to a rich mixture. A routine test consisted of setting the flame to a rich mixture, the To measure the output voltage of the sensor, to suddenly switch the flame to a lean mixture and to scan the oscilloscope at the same time to record the changeover or switchover of the sensor from a rich to a lean mixture, then suddenly switch the flame back to a rich mixture, trigger the oscilloscope again to see the change in output voltage of the probe and finally adjust the flame to a lean mixture and the output voltage of the To measure the probe. The changeover time is the time that the output voltage takes after recording on the oscilloscope needs to go through the range between 600 and 300 mV. When the output voltage of the sensor with a rich mixture ratio is less than 600 mV, the changeover reaction time cannot be determined according to the standards for this changeover reaction measurement will. Output voltage measurements for rich mixtures were then connected to various known values for the ohmic shunt Sensor terminals carried out. These measurements provided data for calculating the internal resistance of the sensors.

030013/0662

A number of thimble-shaped dry gas probe fills have been made for the following examples of spherically twisted zirconia, ytter earth and alumina in proportions of 80%, 14% and 6% by weight prepared by isostatically pressed into the same thimble-like columnar shape and fired at a high temperature.

Example I.

With three rows of dry filling thimbles (AEN-1, AEN-2 and AEN-3), the inner electrode was applied to the inner surface by coating with a platinum suspension, which acts as an adhesive Contained glass frit. Then the thimble with its inner electrode heated in an oxidizing atmosphere to burn off the organic components of the suspension and the platinum to the Zirconia surface to be cemented. Then became the outer platinum catalyst electrode vapor-deposited onto the outer surface of the thimble in a known manner. The outer catalyst layer was used for protection covered with a porous ceramic covering. Then the thimbles were shaped into measuring sensors and checked for their output voltage, their changeover reaction and their internal resistance are checked as described above. The results of the measurements are in Table I listed under the heading "no treatment".

Then the thimbles were subjected to chemical treatment by that on their inner surface an aqueous solution of normal hydrochloric acid was applied by the inner part of the thimbles was filled with the acid. The sensors were kept at a temperature of 50 ° C. for a period of 30 minutes and the Acid solution was removed, whereupon the interior of the probe element was washed out with distilled water and held for at least

030013/0662

2934837

Was dried at 100 ° C for 1 hour. Then the output voltage, the reversed connection and the internal resistance of these sensor elements were measured again. The results of these measurements are in
Table I listed under the heading "After Chemical Treatment". After making these measurements, the sensing elements were subjected to current activation as follows. The sensor elements were used as sensors in a protective housing and with connection lines in an intake port or a duct, the outer surface of the sensor element, which was already coated with the conductive outer catalyst coating, the current

a reducing gas of 0.5% carbon monoxide in nitrogen (with

3 3

0.01 mg / cm water vapor) at a throughput of 710 cm / min. During a 10 minute period the elements were
preheated to about 700 ⁰ C. The internal catalyst conductive electrode was in contact with air and the temperature of the probe
was tapped at the bottom of the interior of the sensor element. The probes were then one for a 10 minute period

Exposed to direct current, with the outer electrode acting as the anode for the

DC charging at a current density of about 167 mA / cm of the

flat surface of the outer electrode or continued gas flow. The direct current was then switched off, followed by a recovery period of 10 minutes at the temperature mentioned above and the outer electrode was still further exposed to the gas flow.

Thereupon the output voltage, the changeover behavior, became again and the internal resistance of these sensor elements is checked. The results of these measurements are in Table I under the heading "after chemical treatment and current activation" listed.

030013/0662

TABEL

O CO O O

O CD CO IS}

Probe treatment HC1 Starting
tension skinny
(mV) 35O ° measurement MF (kOhm) 40 35 800 skinny ° measurement MF (OnTn) I. HC1 fat -306 Umstellver
keep (ms) 40 35 Starting
tension (mV) (ms) I. HC1 (mV) - 94 FM n / f ⁺ 50 39 fat ! 54 Umstellver
keep 30th 125 30th (ms) n / f > 1,000 (mV) 44 FM 20th 121 AEN-1 no treatment 339 n / f ⁺ n / f > 1,000 800 33 (ms) 20th 57 AEN-2 no treatment 330 n / f > 1,000 787 25th 65 AEN-3 no treatment 160 n / f 778 25th + cannot be determined 150 After chemical treatment 55 20th 35 906 147 > 1000 56 25th 50 AEN-1 909 > 1000 805 59 35 45 AEN-2 901 > 1OOO 801 40 45 AEN-3 807 25th 40

After chemical treatment and current activation

AEN-1
AEN-2
AEN-3

Gl.-stream 858 - 16 Gl.-stream 947 13 Gl.-stream 884 - 13

80 40 79 80 40 23 70 40 50

816 43 25th 15th 24 818 58 25th 15th 15th 816 50 25th 15th 20th

NJ CD CO .P - CD U)

The measurement results in Table I show the effect of the chemical treatment on the output voltage and the internal resistance of the Sensor element and also the effect of the combined chemical treatment and current activation on the element with accordingly high output voltage, low internal resistance and significantly shortened changeover time.

Example II

three other thimbles from the series of dry filling thimbles External electrodes (AP7-11, AP7-12 and AP7-13) were applied as in Example I. Then the three sensor elements became chemical treated by applying a 2N aqueous solution of hydrochloric acid (2g per liter of solution) to the inner surface. The inner part the thimble was filled with acid to the point where it covered the inner electrode, and the probe for half an hour heated to 50 ° C and after pouring out the acid, the inner part was rinsed twice with methanol. Then the output voltage, the changeover behavior and the internal resistance of these three elements were measured. The measurement results are given in Table II under the heading "chemically treated" listed. Then these three sensing elements became the current activation by inserting them into an intake manifold subjected to the conductive outer catalyst coating being exposed to the flow of a nitrogen atmosphere (710 cm / min), while the elements were heated to 750 ° C for 10 minutes. At a temperature of 750 ° C and the nitrogen atmosphere A direct current was applied to the outer electrode for 10 minutes, the current density of which is listed in Table II with the outer electrode serving as an anode. Then the direct current was switched off, the sensors were allowed to stay for 10 minutes

030013/0662

Rest long with the conductive outer catalyst electrode remaining at the high temperature to the nitrogen flow. Then the 4 sensor elements measured again. The measurement results are in Ta-Delle II under the head "after chemical treatment and current activation" listed.

- 17 -

030013/0662

TABEL

II

Probe treatment Starting
tension skinny 35O ° measurement MF (koÄm) 85 165 57 Starting
tension skinny 800 measurement 63 25th MF R.
(Onrn) I. fat (mV) Conversions
keep (ms) 65 45 27 fat (mV) Conversions
keep 77 15th (ms) 1 (mV) 229 FM 60 600 55 21st (mV) 89 FM 78 15th 45 -J 885 191 (ms) 55 56 808 88 (ms) 25th 39 I. AP7-11 formerly 887 276 8,400 70 25th 807 85 30th 55 38 AP7-12 chem.beh. 857 6,800 70 820 25th 38 030 AP7-13 chem.beh. 7,900 After chemical treatment and 30th ο Power activation co 25th ·> *
O
cn Current density
mA / cm 748 -132 15th 13th CT)
NJ AP7-11 100 936 35 799 15th 11 AP7-12 20th 944 122 806 13th AP7-13 8th 792

- is - '-U837

As the measurement results in Table II show, already causes one

Current density of only 8mA / cm during a 10 minute period 750 C a shortening of the response time of the sensor element after

the method according to the invention, although with current densities of 20

2
or 100 mA / cm greater improvements can be achieved.

Example III

With four more rows of dry filling thimbles (AP7-10, AP7-21, AP7-22 and AP7-23) the inner and outer electrodes were applied according to the pattern of Example I. These four sensor elements were then chemically treated according to the procedure of Example II. They were then measured. The measurement results are in Table III listed under the heading "chemically treated". Then the four probe elements were made according to the procedure

of Example II subjected to current activation, with the exception of

that the current density was now 100 mA / cm for each element. In AP7-23 the temperature was 600 ⁰ C. The casting time of the direct current as well as the recovery time probe were varied for the four, and these values are listed in Table III. The output voltage, the changeover behavior and the internal resistance of the four sensor elements were then measured again. The measurement results are listed in Table III under the heading "After chemical treatment and current activation".

- 19 -

030013/0662 originally inspected

TABEL

III

Probe treatment 35O ° measurement Starting
tension (mV) Conversions
keep MF R.
(kohm) 30th 17th Starting
tension skinny 800 ° -messuncr 77 15th Mi = (Onm) fat lean 237 PM (ms) 30th 15th bold ι (mV) Conversions
keep 80 20th (ms) (mV) 224 (ms) 70 50 27 (mV) 91 FM 88 20th 45 885 186
269 8,900 70 44 40 28 796 82 (ms) 87 15th 50 36 AP7-1O brazen.beh. 782 After < 6,200 70
120 102 811 83
86 15th 45
55 44 AP7-21 formerly 789
79 7 3,900
7.300 107
106 801
806 25th 43
40 0300 ' AP7-22
AP7-23 formerly
formerly 53 chemical treatment 20th
30th co 42 and power activation 15th σ
he. +) ++) 956 89 50 15th 10 AP7-1O 0.1 10 957 94 40 798 15th 12th AP7-21 0.1 O 932 390 799 15th 20th AP7-22 0.03 O 915 180 784 16 AP7-23 0.1 O 79 3

+) Switch-on time of the current (min) ++) Recovery time (min)

Ca) CD

As the measurement results show, there is no need for a recovery period if the electricity is only applied for a short time, for example 0.1 or 0.03 minutes, and the sensor elements do not seem to need stabilization after this treatment To give sensor elements with significantly improved properties.

Example IV

The inner and outer electrodes of seven other thimbles the range of dry filling thimbles (AP7-15, AP7-14, AP7-16, AP7-17, AP7-19, AP7-18 and AP7-2O) were applied according to Example I. and chemically treated according to the procedure of Example II. Then the output voltage, the changeover behavior and the internal resistance of these seven sensor elements. The measurement results are shown in Table IV under the heading "chemically treated" listed. The seven sensor elements were then current activated according to the procedure of Example II, except that the Activation temperature, the current density and the active time of the direct current were varied for certain sensor elements (see Table IV). In each case there was a warm-up time and a recovery time adhered to for 10 minutes. The seven sensor elements were then measured again. The measurement results are in Listed in Table IV under the heading "After Chemical Treatment and Current Activation".

- 21 -

030013/0662

TABEL

IV

Probe treatment 350 measurement skinny
(mV) Conversions
keep SE
(ms) R.
(köhm) Starting
tension skinny
(mV) 800 ° measurement MF
(ms) R.
(OÄm) I. Starting
tension 176 üä
(ms) 70 fat
(mV) 84 Conversions
keep 35 to
> fat
(mV) 196 6,500 70 61 796 78 (ms) 50 39 I. AP7-15 formerly treated 8 & 0 247 5,500 65 17th 787 89 20th 45 50 AP7-14 formerly treated 893 245 4,600 90 97 803 86 15th 45 34 AP7-16 formerly treated 802 237 6,000 60 108 808 81 20th 45 42 ο AP7-17 formerly treated 798 212 4,600 60 44 79 7 92 30th 40 39 co AP7-19 formerly treated 854 288 4,100 60 76 767 79 25th 55 35 O AP7-18 formerly treated 818 7,200 After chemical treatment 72 812 and power activation 15th 42 13 / AP7-20 formerly treated 887 25 - D662

Temp. Density time

(° C) 2
(mA / cm) (min) 923 87 170 40 24 794 89 15th 15th 15th AP7-15 600 100 0.1 878 - 1 55 50 24 802 80 20th 15th 10 AP7-14 600 100 10 931 54 85 40 26th 794 84 15th 10 11 AP7-16 600 20th 10 922 99 900 40 21st 788 86 15th 15th 20th AP7-17 600 8th 10 857 121 2,800 50 47 794 85 20th 35 41 AP7-19 450 100 0.1 925 95 720 45 33 774 86 15th 15th 23 AP7-18 450 100 ■ 10 852 142 1, 200 70 49 774 81 20th 25th 24 AP 7-20 450 20th 10

CD CJ ■ £ - ·

The measurement results in Table IV show the influence of different temperatures, current densities and times of action of the current on the sensor properties. Although the temperature of 450 C recorded during the measurement did not result in any serviceable sensors, it should be noted that these probes were equipped with internal electrodes made of molten platinum, which is a glass or other binder and that a temperature at which no flux is used in the inner electrode produces the desired shortening the conversion time would be allocated.

The inventive method offers a combined chemical and current treatment of dry fill gauges, which gauges have improved high output voltage properties with rich mixing ratios, a fast response time and a low internal resistance, with these properties result in the efficient, economical and stable operation of oxygen sensors containing such elements.

030013/0662

Claims (9)

Claims 1. Manufacturing process for an activated oxygen sensor element with an increased output voltage in the case of a rich gas mixture, with a shortened response time for the conversion and a reduced internal resistance, wherein the sensor element has a
Has a dry filling body with a conductive inner catalyst electrode on the inner surface and a conductive outer catalyst electrode on its outer surface, characterized in that it comprises two operations: a) the conductive inner catalyst electrode is brought into contact with an acidic reagent selected from the group of inorganic Acids and acidic salts is selected and b) applying a direct current to the sensing element, wherein
the conductive outer catalyst electrode acts as an anode, 030013/0862 ORiGiNAL INSPECTED - ² - 2334637 while in a non-oxidizing atmosphere Temperature of over 450 ° C, and the current density of the catalyst electrode at least 5 mA / cm of the flat surface of the conductive outer catalyst electrode. 2. Manufacturing method according to claim 1, characterized in that
that the non-oxidizing gas is selected from a group of reducing gases, neutral gases and noble gases and mixtures of these gases. 3. Manufacturing process according to claim 2, characterized in that that the non-oxidizing gas is a reducing gas. 4. Manufacturing method according to claim 2, characterized in that the non-oxidizing gas is a neutral gas. 5. Manufacturing method according to claim 4, characterized in that that the neutral gas is nitrogen. 6. Manufacturing method according to claim 1, characterized in that that the current density is between 20 and 150 mA / cm of the flat surface of the conductive outer catalyst electrode. 7. Manufacturing method according to claim 1, characterized in that the high temperature is between 600 and 900 ^° C. 8. Manufacturing method according to claim 1, characterized in that the direct current for a period of time between 6 seconds and 030013/0662 min is created. 9. Manufacturing method according to claim 1, characterized in that that the sensor element after exposure to the current for a Period of time is held at this temperature. "OK. Oxygen sensor element, characterized in that it is after the method of claim 1 is manufactured. 030013 / 0bö2