WO1990004171A1

WO1990004171A1 - SENSOR ELEMENT FOR LIMIT CURRENT SENSOR FOR DETERMINING THE μ VALUE OF GAS MIXTURES

Info

Publication number: WO1990004171A1
Application number: PCT/DE1989/000625
Authority: WO
Inventors: Hermann Dietz; Werner Grünwald; Claudio De La Prieta
Original assignee: Robert Bosch Gmbh
Priority date: 1988-10-14
Filing date: 1989-10-03
Publication date: 1990-04-19
Also published as: ES2016207A6; KR900702364A; DE3834987C2; DE3834987A1; AU4328489A

Abstract

A sensor element for limit current sensors used to determine the μ value of gas mixtures, in particular exhaust gases of internal combustion engines, has outer and inner pump electrodes arranged on a O2- ion conductive solid electrolyte. The inner pump electrode is exposed to the measurement gas supplied through a diffusion slot and the diffusion slot is covered by the solid electrolyte. The solid electrolyte that covers the diffusion slot is a solid electrolyte layer obtained by screen printing a ceramic support. A process is disclosed for producing this sensor element. By covering the diffusion slot with a screen printed solid electrolyte layer, the production of sensor elements for limit current sensors is considerably simplified and pumping cells with very small inner resistance can be obtained.

Description

Sensor element for limit current sensors for determining the λ value of gas mixtures

State of the art

The invention is based on a sensor element for limit current sensors according to the preamble of the main claim. In such sensor elements, which operate according to the diffusion limit current principle, the diffusion limit current is measured at a constant voltage applied to the two electrodes of the sensor element. This current is linearly dependent on the oxygen concentration in an exhaust gas produced during combustion processes, as long as the diffusion of the gas to the pump electrode determines the speed of the overall reaction taking place. It is known to construct such sensors, which operate according to the polarographic measuring principle, in such a way that both the anode and the cathode are exposed to the gas to be measured, the cathode having a diffusion barrier in order to achieve work in the diffusion limit current range. The known limit current sensors are usually used for

Determination of the λ value of exhaust gas mixtures, which denotes the ratio "total oxygen / oxygen required for complete combustion of the fuel" of the air / fuel mixture burning in a cylinder. Due to a simplified and inexpensive method of production, the production of probes and sensor elements which can be produced using ceramic foil and screen printing technology has become established in practice in recent years.

In a simple and efficient manner, planar polarographic probes can be started from platelet-shaped or foil-shaped oxygen-conducting solid electrolytes, for example. made of stabilized zirconium dioxide, which are coated on both sides with an inner and outer pump electrode with associated conductor tracks. The inner pump electrode is advantageously located at the end of a diffusion gap

or biffusion channel through which the measuring gas can reach the electrode. and which serves as a gas diffusion resistor.

From DE-OS 35 45 759 and EP-A 0 142 992, 0 142 993, 0 188 900 and 0 194 082 sensor elements and detectors are also known, which have in common that they each have a pump cell and a sensor cell which consist of platelet-shaped or foil-shaped oxygen-conducting solid electrolytes and two electrodes arranged thereon and have a common diffusion gap or diffusion channel.

A certain disadvantage of known polarographic probes and sensor elements is that the front part of the inner pump electrode facing the supplied measuring gas is subjected to greater stress than the rear part of the pump electrode facing away from the supplied measuring gas. This leads to a high electrode polarization, which requires a high pump voltage. The latter in turn harbors the risk of electrolyte decomposition in the area of the inner pump electrode.

In DE-OS 37 28 618 it is therefore proposed to use a sensor element for polarographic probes to determine the λ values of gas mixtures with a plate which conducts on O ^2- ions or film-shaped solid electrolytes arranged outer and inner pump electrodes, of which the inner pump electrode on the platelet-shaped or f-shaped solid electrolyte in one

Diffusion channel for the measuring gas is arranged, as well as with conductor tracks for the pump electrodes, in the diffusion channel on the side opposite the inner pump electrode to arrange at least one second inner pump electrode which is short-circuited with the first inner pump electrode.

A disadvantage of the known sensor elements, in particular by laminating several solid electrolyte foils together

Laminating films based on stabilized ZrO ₂ , is that their manufacturing process is relatively complicated and their internal resistance is still relatively.

Advantages of the invention

The sensor element according to the invention with the characterizing features of the main claim has significant advantages over the known planar sensor elements. So it is with less

Process steps can be produced more cheaply and easily, since no laminating process is required for its production and a sealing frame is not required. The saving of a sealing frame also enables a narrower design. Another significant advantage is that the solid electrolyte layer can be significantly thinner than the sensor elements made from solid electrolyte foils, thereby reducing the internal resistance. This enables probe operation at lower temperatures, which means that less heating power is required and the probe life is increased.

The sensor element according to the invention can be used in place of known sensor elements with a planar structure in limit current sensors of conventional design. Broadband sensors come into question (for λ≥ 1) Lean sensors (for λ> 1). The sensor element according to the invention can thus be designed solely as a pump cell, optionally with a heating element, for. B. as a lean sensor for diesel engines, and as such in a conventional sensor housing, for. B.

of the type known from DE-OS 52 06 903 and 35 37 051 installed and for measuring the air-fuel ratio in one

lean or rich exhaust gas can be used. However, in addition to the pump cell, the sensor element according to the invention can also have a sensor cell (Nernst cell), which is provided with an additional air reference channel and one of them

Electrode is arranged in the area of the pump electrode in the diffusion channel of the pump cell and the other electrode is located in the

Air reference channel is located.

drawing

In the drawing, two advantageous forms of a sensor element according to the invention are shown, for example.

Show it:

Figure 1 is a schematic cross section through the electrode part of a first embodiment of a sensor element according to the Invention with rectangular electrodes.

2 shows the layout of a sensor element with that in FIG. 1

illustrated electrode part;

3 shows a schematic cross section through the electrode part of a second embodiment of a sensor element according to the invention, in which the electrodes are arranged in a ring around the central measurement gas inlet opening;

Fig. 4 shows the layout of a sensor element with the electrode part shown in Fig. 3. The first embodiment of a sensor element according to the invention, shown schematically in FIGS. 1 and 2, consists of the carrier or substrate 1, the diffusion gap 2, the inner pump electrode (cathode) 3 with the associated conductor track 3 '

Solid electrolyte layer 4, the outer pump electrode (anode) 6 with associated conductor track 6 ', the insulation layer 5, the engobe 8 and the cover 9. The sample gas entry occurs at 7.

The second embodiment of a sensor element according to the invention, shown schematically in FIGS. 3 and 4, consists of the carrier or substrate 1, the diffusion gap 2, the inner pump electrode (cathode) 3 arranged in a ring around the central measuring gas inlet opening 7 with the associated conductor track 3 ', the solid electrolyte layer 4, the outer pump electrode (anode) 6 arranged in a ring around the central measurement gas inlet opening 7 with the associated conductor track 6 ', the insulation layer 5, the engobe 8 and the cover 9.

The carrier or substrate 1 of the sensor elements according to the invention consists of a ceramic material, as is usually used for the production of sensor elements, for example based on ZrO ₂ or Al ₂ O ₃ . It has proven to be advantageous to use films made of unsintered ceramic material with a layer thickness of 0.3 to 2.0 mm, in particular of approximately 1.0 mm, to produce the sensor elements.

If the coating composition printed on the carrier 1 to produce the diffusion gap 2 is decomposed without residue at pre-sintering or sintering temperature, evaporated or burned, then it has no filling. However, the diffusion gap advantageously has a filling of coarsely porous sintered ceramic

Material, e.g. B. based on Al ₂ O ₃ or ZrO ₂ . It has proven to be advantageous to use a porous sintered ceramic material with a thermal expansion behavior to generate the diffusion gap, which de the expansion behavior used solid electrolyte layer 4 corresponds or at least comes close. The porosity of the filling can be created by adding pore formers which burn, decompose or evaporate during the sintering process. Typical pore formers that can be used are e.g. B. thermal carbon black, plastics, e.g. B.

on polyurethane base, salts, e.g. B. ammonium carbonate and organic substances, such as. B. theobromine and indanthrene blue.

Such pore formers are added to the porous sintering starting material in such an amount that a material with a porosity of z. B. 10 to 50%. The middle one

Pore diameter, which can be determined by the particle size of the pore former used, is preferably included

about 0.1 to 10 μm.

According to a further advantageous embodiment of the invention, the diffusion gap 2 has a filling that both a Knudsen and a gas phase diffusion take place. This means that in front of the inner pump electrode 3 acts as a diffusion barrier for the sample gas channel system for a

Mixed diffusion from Knudsen and gas phase diffusion can be arranged, as described in more detail in DE-OS 37 28 289.

The pump electrodes 5 and 6 preferably consist of a

Platinum group metal, especially platinum, or alloys of platinum group metals or alloys of platinum group metals with other metals. They may contain a ceramic scaffold material, e.g. B. in the form of a YSZ powder, with a volume fraction of preferably about 40 vol%. They are porous and as thin as possible. They preferably have a thickness of 8 to 15 μm. The conductor tracks belonging to the pump electrodes preferably also consist of platinum or a platinum alloy of the type described. They can also be produced from a paste based on a precious metal cermet. Pastes suitable for printing on the electrodes and conductor tracks can be made in a known manner using organic ones

Binders and / or adhesion promoters, plasticizers and organic solvents are produced. If at the same time insulating intermediate layers are to be produced, then smaller amounts of compounds with a 5-valent or higher-valent cation can be added to the pastes, e.g. B.

Nb ₂ O ₅ . As adhesion-improving additives are z. B. Al ₂ O ₃ or ZrO ₂ .

An advantageous precious metal cermet paste for producing a conductor track can thus consist, for example, of: 85 parts by weight of Pt powder (3 m ² / g)

12.5 parts by weight of Nb ₂ O ₅ powder (8 m ² / g)

2.5 parts by weight of Al ₂ O ₃ powder (10 m ² / g)

as well as binders, plasticizers and solvents.

The solid electrolyte layer 4 consists of one of the known oxides of tetravalent metals used for the production of O ^2- ion conductive solid electrolyte foils, such as in particular ZrO ₂ , CeO ₂ , HfO ₂ and ThO ₂ with a content of divalent alkaline earth oxides and / or preferably trivalent oxides Rare Earth. Typically, the layer can contain about 50 to 97 mol% of ZrO ₂ , CeO ₂ , HfO ₂ or ThO ₂ and 50 to 3 mol% of CaO, MgO or SrO and / or rare earth oxides and especially Y. ₂ O ₃ exist. Advantageously, there is

Layer of ZrO ₂ stabilized with Y ₂ O ₃ . The thickness of the layer can advantageously be 10-200 μm, in particular 15 to 50 μm.

The pastes used to produce the solid electrolyte layer can also be produced using binders and / or adhesion promoters, plasticizers and organic solvents. An advantageous paste for producing the solid electrolyte layer has, for. B. The following composition: 40 g zirconium dioxide and

10 g of a preliminary solution

12.50% by weight of polyvinyl butyral,

3.91% by weight of dibutyl phthalate,

80.59% by weight of butyl carbitol and

3.00% by weight sebacic acid butyl ester.

The insulation layer 5, which insulates the conductor 6 'of the outer pump electrode 6 from the solid electrolyte layer 4, consists of an insulating layer, for. B. based on Al ₂ O ₃ , as they are usually produced in the manufacture of planar sensor elements, to isolate conductor tracks from a solid electrolyte. The insulation layer 5 can be 15-20 μm thick, for example.

If necessary, such an insulation layer can also

be arranged between the carrier 1 and the conductor track 3 'of the inner pump electrode 3, for. B. in the case where the

Carrier is a carrier based on solid electrolytes, for example a ZrO ₂ carrier. However, the arrangement of such insulation layers is not absolutely necessary, ie they can

also be omitted.

The engobe or protective layer 8 is porous and consists, for example, of a layer based on Al ₂ O ₃ or Mg spinel, as is usually used in planar sensor elements for covering electrodes. The strength of the engobe lies

for example at 10 - 40μm.

According to an advantageous embodiment of the invention, the porous engobe consists of an Al ₂ O ₃ and / or Mg spinel matrix with ZrO ₂ particles of the type known from DE-OS 37 37 215 embedded therein.

Finally, the cover 9 can be constructed from the same material as the engobe. As a rule, however, a somewhat fine-grained material is used to produce the cover. example

To produce a sensor element of the type shown schematically in FIGS. 3 and 4, a film made of zirconium dioxide stabilized with yttrium and having a layer thickness of 0.5 mm was used as the carrier. The diffusion gap was introduced in thick-film technology through a screen printing layer made from a mixture of theobromine and coarse-grained ZrO ₂ with a grain size of 10 μm, the theobromine during the later sintering process in the temperature range around 300 ° C., leaving about

30 μm high and 1.3 mm deep annular gap evaporated. The ZrO ₂ solid electrolyte layer was produced by printing on a paste of ZrO ₂ stabilized with Y ₂ O _{3 and} having a particle size of ~ 1-2 μm. The printed layer had a thickness of 80 vm. The pump electrodes, which consist of platinum, were also applied using known screen printing technology, with the outer electrodes being applied to them

Pump electrode-carrying surface of the solid electrolyte layer in the area of the conductor track of the outer pump electrode beforehand

10 μm thick Al ₂ O ₃ insulating layer was applied. The ring-shaped. Pump electrodes had an outer diameter of 2.8 mm and an inner diameter of 1.4 mm with a thickness of 12 μm.

The conductor tracks were produced from a conventional Pt cermet paste from 85 parts by weight of Pt powder and 15 parts by weight of YSZ powder.

A paste was applied to create the ring-shaped engobe

Al ₂ O ₃ base printed. The engobe had a thickness of approx.

30 μm.

The cover was also based on a paste

Al ₂ O ₃ base printed. It had a thickness of approx. 10 μm.

The central sample gas inlet opening had a diameter of 0.25 mm. After the electrodes, conductor tracks, insulating layer, engobe and cover had been applied, the coated carrier was subjected to a sintering process in which it lasted for about 3 hours was heated to a temperature in the range of 1380 ° C.

The sensor element produced was inserted into a housing of the type known from DF-OS 32 06 903 and used to determine the λ value of gas mixtures. Excellent reproducible results have been obtained.

Preferably, the manufacture of a sensor element according to the invention is carried out mechanically in multiple use. The width of the probe is advantageously approximately 4 to 6 mm. The

Electrode diameter is advantageously 3 to 4 mm, z. B. 3.6 mm.

Claims

Expectations

1. Sensor element for limit current sensors for determining the

λ value of gas mixtures, especially the exhaust gases from

Internal combustion engines with an O ^2- ion conducting

Solid electrolytes arranged outer and inner pump electrodes, of which the inner pump electrode is accessible for the measurement gas supplied through a diffusion gap and the diffusion gap is covered by the solid electrolyte, and with conductor tracks for the pump electrodes, characterized in that the cover for the

Diffusion gap-forming solid electrolyte is a solid electrolyte layer produced by printing a ceramic carrier using screen printing technology.

2. Sensor element according to claim 1, characterized in that the solid electrolyte layer is a layer of ZrO ₂ stabilized with Y ₂ O ₃ .

5. Sensor element according to claim 1 or 2, characterized in that the layer thickness of the solid electrolyte layer is 10 - 100 microns.

4. Sensor element according to one of claims 1 to 3, characterized

characterized in that the diffusion gap is filled with a coarsely porous sintered material based on Al ₂ O ₃ or ZrO ₂ .

5. Sensor element according to one of claims 1 to 4, characterized

characterized in that the pump electrodes are arranged in a ring around a central measuring gas inlet opening.

6. Sensor element according to one of claims 1 to 5, characterized

characterized in that the carrier consists of a film based on ZrO ₂ or Al ₂ O ₃ ceramic.

7. Method for producing a sensor element according to a

of claims 1 to 6, characterized in that a mass is first placed on a support made of unsintered ceramic

to form the diffusion gap, then the inner one

Pump electrode with associated conductor track, over the inner one

Pump electrode a solid electrolyte layer with gas inlet opening, an insulation layer, the outer pump electrode

with associated conductor track, an engobe and finally a cover using screen printing technology and that the

printed carrier with formation of the diffusion gap

sinters.

8. The method according to claim 7, characterized in that one

to form the diffusion gap, a mass is imprinted on the carrier, which decomposes or burns during the sintering process.

9. The method according to claim 7, characterized in that one

to form the diffusion gap on the carrier

Imprints mass that sinters porous during the sintering process.

10. The method according to claim 9, characterized in that one

used a mass consisting essentially of a mixture of finely divided Al ₂ O ₃ or optionally stabilized

ZrO ₂ and a pore former.

11. The method according to claim 10, characterized in that

as a pore former which evaporates during the sintering process or

burning substance used.

12. The method according to any one of claims 7 to 11, characterized in that sintering the printed carrier by heating for 2 to 6 hours at a temperature of 1340 to 1400ºC