Resistance Probe for Determining Gas Compositions and Method of Producing It

The invention is based on a generic resistance probe. Such resistance probes are known, for example, from German Patent 2,908,916, in which a heating conductor, electrodes, a semiconductor layer as well as a gas-permeable cover layer, separated by insulating layers, are applied in multi-layer technology onto a ceramic carrier plate. Experiments have shown that such protective layers do not exhibit satisfactory characteristics with respect to adhesion and reproducibility.

In contrast thereto, the resistance probe according to the invention comprises the following layers: an insulating ceramic substrate, a superposed pair of electrodes, a superposed semiconductor layer, a superposed ceramic covering, and at least one covering sheet disposed on the pair of electrodes and containing at least one recess in which the semiconductor layer with the ceramic covering is disposed on the pair of electrodes and which allows a gas mixture to pass through to the ceramic cover. The semiconductor layer functions as a gas sensor and is in electrical contact with the pair of electrodes. The ceramic covering is configured as an engobe protective layer with a plurality of pores which allow the gas mixture to pass through to the semiconductor layer.

A second embodiment of the invention comprises the following layers: an insulating ceramic substrate, a superposed pair of electrodes, a superposed semiconductor layer, and a superposed ceramic covering. The semiconductor layer functions as a gas sensor and is in electrical contact with the pair of electrodes. The ceramic covering is configured as a protective layer structure produced by thick-film technology in the form of a pattern of regions covering the semiconductor layer and with openings which allow access for the gas mixture to the semiconductor layer. The Invention has the advantage of its layer structure having sufficient stability under permanent stress. On the one hand, the structure according to the invention delays chemical aging of the resistance layer in that corrosion of the resistance layer, that is configured as a semiconductor layer, due to contaminants from the exhaust gas is prevented and, on the other hand, mechanical aging caused by abrasion of the resistance layer due to particles in the stream is also delayed. Even after long periods of operation, release of the semiconductor layer from the carrier is not observed.

Moreover, the application methods for the porous engobe protection layer, namely screen-printing, insertion of a porous sintering film or dripping in a suspension, are less expensive than the plasma spraying method employed in the past.

The method according to the invention is distinguished in that initially a sensor carrier body equipped with electrodes and an integrated heating element is prefabricated and sintered at high temperatures, whereupon a semiconductor layer is then applied as the resistance layer. This layer possibly carries an engobe protective layer and is sintered in together with the latter at lower temperatures. The advantage of this method is that the engobe protective layer is unable to react with the semiconductor layer during the sintering process as well as during later operation of the sensor and is therefore sufficiently stable.

Another advantageous method includes the application of a protective layer structure provided with openings only in certain regions so as to ensure access of the measuring gas to the semiconductor layer. The adhesion of the protective layer structure can be further improved by pressing it into the semiconductor layer before the sintering.

Two embodiments of the invention are illustrated in the drawings and will be discussed in greater detail in the description below.

FIG. 1 depicts the structure of a resistance probe according to a first embodiment of the invention.

FIG. 2 depicts an alternative to the first embodiment.

FIG. 3 depicts the layout of a resistance probe according to the invention.

FIG. 4 depicts the layout of a further embodiment.

FIGS. 5A and 5B depict the production of a preferred embodiment; and

FIG. 6 depicts an advantageous protective layer structure.

Unless otherwise mentioned, all layers of the resistance probe according to the invention are applied in a screen-printing process.

The sensor carrier body 1 of FIG. 1 is prefabricated in multi-layer technology from materials and according to process steps as disclosed in German Patent 2,908,916. For this purpose, a meander-shaped heating conductor 5 is applied to a thin film 4 of organic material and of an insulating ceramic material, preferably of aluminum oxide including more than 90 volume % Al.sub.2 O.sub.3. On top of it are applied, separated by a second thin sheet 6, electrodes 7 and then at least one aluminum oxide containing ceramic cover layer 8. Cover layer 8 is provided with a recess or opening 9. The individual layers are dried intermediately if required and/or pre-sintered. The entire carrier body 1 is then sintered at a temperature of at least 1550.degree. C.

The intermediate drying of carrier body 1 may also be omitted if the layer sequence has sufficient stability for the sintering process without being intermediately dried.

In a next process step, a semiconductor layer 2 in the form of a suspension or a screen-printing paste is introduced into the recess 9 provided for this purpose in cover layer 8. After intermediate drying of layer 2, if required, a porous engobe protective layer 3 is applied on top of it. Different methods are possible to accomplish this. Layer 3 may be produced, for example, by dripping in an engobe suspension, by printing on a screen-printing paste, or by inserting a thin, porous sintering sheet.

In the last process step, the resistance probe is sintered at 1150.degree. to 1400.degree. C.

According to a preferred embodiment, the sintered carrier body 1 may initially be supplied with a semiconductor layer 2 which is sintered in at 1150.degree. to 1400.degree. C. Only then is the engobe protective layer 3 applied and sintered in at 1150.degree. to 1400.degree. C.

In the sintered state, the engobe layer is composed of aluminum titanate or mixtures thereof with metal oxides. Applicable oxides are, for example, titanium dioxide, zirconium dioxide, yttrium stabilized zirconium dioxide, magnesium spinel or zirconium titanate. Mixtures of aluminum titanate and titanium dioxide preferably containing less than 50 volume percent titanium dioxide have been found to be particularly advantageous. The engobe composition according to the invention is selected in such a way that no undue reaction with semiconductor layer 2 takes place, neither during production nor during operation of the probe. For example, the aluminum titanate may be ATG made by Dynamit Nobel, which contains stabilizers in addition to aluminum oxide and titanium dioxide and which is composed, for example under the name "Aluminium Titanate ATG-3", in weight percent, of 53.8% Al.sub.2 O.sub.3, 32.75% TiO.sub.2, 3.0% ZrO.sub.2, 7.9% SiO.sub.2, 2.1% MgO. 0.2% Fe.sub.2 O.sub.3, 0.2% Na.sub.2 O and less than 0.05% CaO.

To improve the layer strength of the engobe according to the invention, the addition of small percentages of silicate containing fluxing agents has been found to be particularly advantageous, for example, in the form of SiO.sub.2 -TiO.sub.2 -ZrO.sub.2 glass. The addition of the fluxing agents may also be effected in the form of metal-organic compounds such as, for example, alkoxides, propyl titanate, propyl zirconate, butyl titanate or butyl zirconate. It is preferable to use fluxing agents in the engobe layer which correspond to the fluxing agents of the ceramic substrate.

To ensure the gas access to the semiconductor layer the engobe must of necessity be porous. This is enhanced by the addition of pore formers, such as, for example, soot, theobromine, indanthrone or polywaxes, to the engobe suspension or paste.

FIG. 2 shows an alternative to the embodiment shown in FIG. 1, with the particularity that, to accommodate the semiconductor layer 2 and the engobe protective layer 3, respectively, cover layer 8 and 8', respectively, are provided with corresponding recesses. As can be seen in FIG. 1 and FIG. 2, cover layer 8 may be provided, for example, with a larger square recess 9 and cover layer 8' with several recesses 9' having a square cross section, all arranged above recess 9. Otherwise, this embodiment can be produced in the same manner as described in connection with FIG. 1.

In contrast to the embodiment shown in FIG. 1, the variation of FIG. 2 is provided, in an advantageous manner, with an improved mechanical strength thanks to the grid-shaped configuration of cover layer 8'.

FIG. 3 depicts a layout for a further embodiment of the resistance sensor according to the invention. An insulating substrate 4 is composed of an aluminum film provided with connecting holes, with the film containing 90% aluminum oxide and having a thickness of 0.5 mm. On its one large surface, the film is provided with platinum cermet contacts 4a and on the other large surface with a platinum cermet heating conductor 5. This is followed by a screen-printed layer of an interlaminary binder 6a composed of 90% aluminum oxide. On a separating sheet 6 having a thickness of 0.5 mm, electrodes 7 are applied which are composed of 90% platinum cermet. The leads to electrodes 7 are covered with a further layer of an interlaminary binder 8a composed of 90% aluminum oxide and one or two covering films which have a thickness of 0.5 mm and are composed of 90% aluminum oxide are applied or glued on top of the binder. Covering layer 8 is provided with a recess 9 and with contacts 7a as well as contacting holes for electrodes 7.

The sensor carrier body prefabricated in this way is sintered at temperatures between 1500.degree. and 1650.degree. C. and then semiconductor layer 2 and possibly the engobe protective layer 3 are introduced into the recess 9 provided for this purpose in cover layer 8 and are sintered in at temperatures between 1150.degree. and 1400.degree. C., preferably at 1200.degree. to 1300.degree. C.

FIG. 4 depicts a preferred embodiment of the resistance probe of FIG. 3. According to this embodiment, an intermediate film 6 is provided with a recess and with slits which are configured in such a way that semiconductor layer 2 is substantially protected against corrosive, erosive and/or cavitative attacks from the exhaust gas. The recess may be lined with a layer 10 of a cavity forming agent such as, for example, theobromine. The semiconductor layer 2 can be introduced into the prefabricated sensor carrier body 1 of FIG. 4 according to various methods: by dripping in a suspension, producing a suction effect from capillary forces, with the substrate surface being pretreated, if required, for example, by cleaning with alcohol or by inserting a porous, sintering titanium dioxide sheet. By means of an interlaminary binder 8a, a covering layer 8 is applied to intermediate film 6 and is provided with a recess 9. Electrodes 7 are connected with contacts 7a and 4a, respectively, by way of contacting holes. One of electrodes 7 may be short-circuited with heating element 5.

FIG. 5A is a sectional view of a further embodiment of a resistance probe according to the invention. Electrodes 7, semiconductor layer 2 as well as the protective layer structure 11 equipped with regions covering semiconductor layer 2 and openings ensuring free access for the measuring gas to semiconductor layer 2 are applied to the insulating substrate 4.

However, the properties of such a probe, particularly its resistance to aging, can be further improved if, as shown in FIG. 5B, the protective layer structure 11 applied to semiconductor layer 2 is fitted into the semiconductor layer 2 before the sintering process, for example by means of a pressing die 12 while employing a separating sheet 13, for example, a siliconized polyester sheet. Pressures of, for example, 20 to 80 bar and advantageously temperatures from room temperature up to the drying temperature of the printed layers can be employed in this case.

FIG. 6 depicts a grid and raster-shaped protective layer structure before it is pressed in.

Aside from the protective layer structure shown in FIG. 6, the most varied other structures are also possible; that is, the openings which give the measuring gas direct access to the semiconductor layer 2 may also be composed, for example, of slits or round or oval openings.

The operation of the resistance probe according to the invention will now be described for the example of a titanium dioxide sensor. It is based on the chemical equilibrium between lattice defects in the titanium dioxide and gaseous oxygen in the surrounding atmosphere. A low oxygen concentration in the atmosphere as, for example, in the case of a fatty mixture or in a vacuum, due to the great difference in oxygen activity between the titanium dioxide and the atmosphere, causes the transfer of the oxygen ions from the titanium dioxide into the atmosphere. Oxygen-caused lattice defects and free electrons are created within the titanium dioxide. Thus, due to the development of free electrons, the resistance of the sensor drops in the fatty region. In the lean region, the opposite reaction takes place, and the resistance of the sensor increases.

Generally, the following equation applies: ##EQU1## where A is a constant, E is the activation energy for the formation of the lattice defect, k is the Bolzmann constant and n a constant which is a function of the nature of the lattice defects. In the temperature range of the oxygen concentration measurement, the value of n is approximately 4 and is based on the conductivity over Ti.sup.3+ locations.

This equation shows that, with a constant ambient temperature, the resistance of the sensor is an exclusive function of the oxygen concentration. However, no chemical or mechanical change in the semiconductor layer must take place which would cause a change in resistance.

To produce a resistance probe according to the invention, a carrier made of a pre-sintered Al.sub.2 O.sub.3 film of a thickness of 0.5 mm was imprinted in a screen-printing process with initially two platinum electrodes. For this purpose, a conventional Pt cermet paste composed of 85 parts per weight Pt powder and 15 parts per weight YSZ powder was employed.

After printing on the electrodes, the structure was laminated and sintered at 1500.degree.-1550.degree. C. A semiconductor layer based on a pasty preparation with TiO.sub.2 of an average grain size of about 0.5 .mu.m was printed onto this prefabricated sensor carrier body.

Then the protective layer structure in the form of a grid or raster as shown in FIG. 2 was printed onto the semiconductor layer. A pasty preparation of the following composition was employed: 70 volume % Al.sub.2 O.sub.3 .multidot.TiO.sub.2, average grain diameter 2.5 .mu.m, 30 volume % TiO.sub.2, as above, and a catalytically active additive of 1 weight % Pt powder, specific surface area 15 m.sup.2 /g according to German Patent 2,265,309.

Seventy percent of the surface area of the semiconductor layer were covered with the pasty preparation. The edge length of the practically square opening was 0.1 to 0.2 mm.

The still damp protective layer structure was then pressed into the semiconductor layer by means of a pressing die.

Following the pressing process, the material was sintered for three hours at a temperature in the range from 1250.degree. to 1300.degree. C. The resistance probe obtained in this way was inserted into a housing of the type disclosed in DE-OS [German unexamined published patent application] 3,206,903 and employed to control the air/fuel ratio in an internal combustion engine.