WO1991010130A1 - Wide-band probe platelet for determining the oxygen concentration in exhaust gases - Google Patents

Abstract

The proposal is for a heatable wide-band probe for determining the oxygen concentration in exhaust gases which consists of a minimum number of ceramic foils of determined composition. Here, because of their Al2?O3? content, the individual ceramic foils have electrical conductivities corresponding to their function and are matched in their sinter and heat expansion behaviour so as to ensure easy manufacture and long useful life.

Description

 Platelet-shaped broadband probe for the determination of the oxygen concentration in exhaust gases

State of the art

The invention relates to a platelet-shaped broadband probe for the determination of the oxygen concentration in exhaust gases, in particular the exhaust gases of internal combustion engines according to the preamble of the main claim.

It is generally known to determine the oxygen content in exhaust gases, in particular from internal combustion engines, sensors or so-called λ-probes with at least one measuring cell, with an electrode exposed to the exhaust gas and an electrode normally exposed to air, the so-called reference electrode, and a solid electrolyte between the two electrodes.

Known sensors of this type are based on the principle of the oxygen concentration chain with ion-conducting solid electrolytes. They exist e.g. B. from a tube closed on one side made of stabilized zirconium dioxide, on the outer surface facing the exhaust gas there is a porous platinum layer which forms the electrode and at the same time catalyzes the greatest possible adjustment of the thermodynamic equilibrium. The electrode exposed to the rental gas is inside the pipe. Such sensors or λ probes of the so-called finger version are e.g. B. from DE-PS 28 52 647 and 29 13 633 and DE-OS 27 11 880 known.

In the case of the so-called single-cell, ie, a Nernst cell λ probes of the type described, the maximum output signal is = 1.02 at approximately 60 mV, as can be seen, for example, from the SAE Technical Paper Series 840141, reprint from the magazine "Sensors and Actuators SP 567, Fig. 11 results.

In the measuring sensor known from DE-PS 27 18 907, both electrodes of the oxygen concentration cell are arranged one behind the other on a massive, electrically insulating carrier body, a surface area of the solid electrolyte being located between the electrodes. From DE-PS 27 18 907 iet ee also already known to manufacture measuring sensors with a plurality of measuring cells connected in series, the individual measuring cells alternately having successive electrode and solid electrolyte areas and being combined into a rod-like measuring cell arrangement. With this configuration, the sensor not only reaches its working temperature very quickly, but also provides an increased output voltage through the series connection of several measuring cells, which is easier to evaluate than the output voltage of a single measuring cell and which can be used to measure a drop in the output voltage due to aging processes.

It is also known to use planar exhaust gas probes to determine the λ value of gas mixtures, which can be produced in a particularly cost-effective manner using ceramic film and screen printing technology. In this regard, reference is made, for example, to EP-A 0 188 900 and EP-A 0 143 993 as well as DE-OS 30 17 947 and 35 43 759. It is useful to use ceramic films based on ZrO ₂ , CeO ₂ , HfO ₂ and ThO _2nd The reason why ZrO ₂ , CeO ₂ , HfO ₂ and ThO ₂ moldings can be used as solid electrolytes is due to the presence of oxygen ion vacancies as a result of the addition of stabilizers, such as, for. B. Ca ²⁺ and Y ³⁺ .

EP-A 0 162 603 and EP-0 168 938 also disclose a layered structure which can be used as limit current probes for determining the oxygen content of exhaust gases. They can each have a pump cell and a sensor cell. Advantages of the invention

The platelet-shaped admission band probe according to the invention, which represents a two-line limit current probe, with the characterizing features of the main claim has the advantage over the known limit current scans having a layer structure that the plates or foils used to manufacture the probe are due to their composition and in particular their Al ₂ O ₃ - Gehaites have the electrical conductivity corresponding to their function and are well matched in their sintering and thermal expansion behavior. Since the probe according to the invention only has as many layers as are absolutely necessary, it is more economical than the known comparable probes of the prior art.

A zirconium oxide ceramic, as is known from DE-OS 28 10 134, is particularly suitable for producing the probe according to the invention. H. a zirconium oxide ceramic with fine-grained and thermally stable structure and with high thermal shock resistance.

In the simplest case, a platelet-shaped broadband probe according to the invention is constructed from:

(a) a base plate with a heating coil applied thereon;

(b) a spacer plate;

(c) a solid electrolyte plate with electrodes applied on both sides and Pt-ZrO ₂ phase boundaries and either

(d) an insulating plate and

(e) a cover plate with electrodes applied on both sides above the solid electrolyte plate or (f) one arranged on the underside of the base plate

Base plate with electrodes. The spacer plate preferably has a U-shape.

The individual plates or foils that form the probe are made of a ZrO ₂ ceramic, which is composed as follows:

Base plate: 20-30 mass%, fully stabilized ZrO ₂ and 80-70 mass% Al ₂ O ₃ ;

Spacer plate: 25-35% by mass of fully stabilized ZrO ₂ and 75-65% by mass of Al ₂ O ₃ or 75-65% by mass of fully stabilized ZrO ₂ and 25-35% by mass of Al ₂ O ₃ ;

Solid electrolyte,

Top and base plate: 90-95% by mass of fully stabilized ZrO ₂ and 10-5% by mass of Al ₂ O ₃ or 90-97% by mass of partially stabilized tetragonal ZrO ₂ as well as 3-10% by mass of Al ₂ O ₃ ;

Insulating plate: 20-30% by mass of fully stabilized ZrO ₂ and 80-70% by mass of Al ₂ O ₃ .

The base plate has a low electrical conductivity due to its addition, so that it can serve as a carrier for a keel winding.

A volistabilized ZrO ₂ , which contains up to 8 mol% of Y ₂ O ₃ , is advantageously used to produce the individual plates. However, other known fully stabilized zirconium dioxides are also suitable.

The partially stabilized tetragonal ZrO ₂ preferably has 4-5 mol% Y ₂ O ₃ .

The individual thin ceramic plates or ceramic foils used for the production of broadband probes according to the invention with a thickness of, for example, 0.1 to 0.6 mm, in particular 0.25 to 0.3 mm, can be printed with the necessary electrodes, associated conductor tracks etc. using known thick-film techniques, Punching operations, laminated together and sintered.

The diffusion measuring gap z. B. can be produced in a known manner by punching or by printing a layer which can be evaporated or burned out during the sintering process. To produce such layers, customary known cavity formers can be used, e.g. B. organic compounds, for example, based on polyurethane, or theobromine, salts, such as. B. ammonium carbonate or thermal carbon black powder.

Via holes can be created by simply punching them out. The isolation of the via holes can e.g. B. by means of an insulating Al ₂ O ₃ layer or by means of a suspension or paste, as used in the iman to form the conductor tracks, but one or more connections with a 5- or higher-valued cation is added, which can be found in the Sintering process subsequent to the lamination process can diffuse into the solid electrolyte substrate at temperatures up to 1600 ° C.

To produce the electrodes of the individual cells and the associated conductor tracks, pastes based on noble metal, in particular platinum or noble metal cermet baeis, in particular platinum cermet baie, are preferably used in thick-film technology. Such pastes can be prepared in a known manner using organic binders and / or adhesion promoters, plasticizers and organic solvents.

For example, the electrodes and associated conductor tracks can consist of a mixture of 60% by volume and 40% by volume of YSZ stabilized with 4 mol% or only Pt. In turn, compounds with a 5- or high-quality cation can be added to the pastes used to form the conductor tracks in order to create isolating regions. The insulation of the conductor tracks or electrode feeds, the electrical connection between the individual electrodes including the plated-through holes and, if present, the heating layer is expediently carried out using a Pt-Nb ₂ O ₅ mixture, such as that described in DE-OS 37 26 479 is known.

The films required to produce a broadband probe according to the invention can be advantageously with interlaminar binders, for. B. on YSZ-3asie together.

The laminate dressing is then sintered, e.g. B. by 1- to 10-hour heating to temperatures of 1350 to 1500 ° C. After, but possibly also before the sintering process is carried out, electrical contact areas can be printed in the area of the through-hole.

drawing

The figures serve to explain the invention in more detail. The following are shown schematically in:

Figure 1 shows the structure of a first embodiment of a broadband probe according to the invention with a circuit diagram.

Fig. 2 shows the structure of a second embodiment of a broadband probe according to the invention with a circuit diagram.

The broadband probe according to the invention shown in Fig. 1 is constructed from the base plate 1 with the heating winding 2, the spacer plate 3, the solid electrolyte plate 4 with the electrodes 5 and 6, the insulating plate 7 with bore 8 and annular channel 9 and the cover plate 10 with bore 11 and Electrodes 12 and 13.

Solid electrolyte plate 4 and cover plate 10 have Pt electrodes and a Pt surface that have a sufficient 3-phase boundary guaranteed. The cover plate 10 and the insulating plate 7 have a bore on the same part, which can have a diameter of 0.4 to 0.7 mm, for example. On its upper side, the insulating plate 7 has an annular channel with a diameter of, for example, 1.0 to 1.5 mm and a depth of 20 to 40 μm. Annular channel 9 and bore 8 are interconnected by a narrow groove. The ring channel 9 forms the diffusion measurement gap of the two-cell broadband limit current probe according to the invention. The electrodes 12 and 13 have corresponding bores 14 and 15, respectively. The holes can be created, for example, by simple punching out and the ring channel by stamping.

The second embodiment shown in FIG. 2 of a broadband probe according to the invention differs from the embodiment shown in FIG. 1 in that a base plate 16 with a recess 17 forming the diffusion gap is attached to the underside of the base plate. In this second embodiment, the diffusion measurement gap is embossed together with the electrode surface, for example 20 to 40 μm deep. The Pt electrodes on the bottom and top of the base plate 16 are in turn applied by means of thick-film technology.

The individual plates or foils which form the platelet-shaped broadband probe according to the invention do not of course need to be rectangular as shown. Rather, they can also be round or oval, for example. The electrodes can also be different, e.g. B. have a round shape. The electrodes can also be protected by insulating layers.

The isolation of the electrodes from the foil substrates and the creation of icing areas between the individual cells can e.g. B. according to the process known from DE-OS 37 26 479, after which a suspension or paste me mincestens a compound with one or different 5- or. On the electrically insulated areas of the solid electrolyte substrate higher-value cations are applied and the 5- or higher-value cations are allowed to diffuse into the solid electrolyte substrate by heating.

The electrical insulation of the conductor tracks from the solid electrolyte foils can also be carried out according to the method known from DE-OS 37 36 479, by using the masses used for printing the conductors or connections or the contacting masses one or more connections with one or different 5- or high-quality cations of the type mentioned.

An interlaminar binder of the known type can be used to laminate the individual plates or foils together.

The laminating and sintering of the joined foils is also carried out in a manner known for the production of planar probes, e.g. B. by heating for 0.5 to 3 hours to a temperature of e.g. B. 1200 to 1400 ° C.

Claims

Claims 1. Platelet-shaped broadband probe for the determination of the oxygen concentration in exhaust gases, characterized by a base plate (1) with an applied thereon Heating coil (2); a spacer plate (3); a solid electrolyte plate (4) with applied on both sides Electrodes (5, 6) and precious metal ZrO ₂ phase boundaries as well as either a leolierplatte (7) and a cover plate (10) with electrodes applied on both sides (14, 15) over the solid electrolyte plate (4) or one arranged on the underside of the base plate (I) Base plate (16) with electrodes (18, 19), where are: the base plate (1): 20-30% by mass of fully stabilized ZrO ₂ and 80-70% by mass of Al ₂ O ₃ ; the spacer plate (3): 25-35% by mass of volatilized ZrO ₂ and 75-65% by mass of Al ₂ O ₃ or 75-65% by mass of fully stabilized ZrO ₂ and 25-35% of mass -% from Al ₂ O ₃ ; the solid electrolyte, the Cover and base plates (4, 10, 16): 90-95% by mass of fully stabilized ZrO ₂ and 10-5% by mass of Al ₂ O ₃ or 90-97% by mass of partially stabilized tetragonal ZrO ₂ and 10-3 Mass% from Al ₂ O ₃ and the Insulating plate (7): 20-30% by mass of fully stabilized ZrO ₂ and 50-70% by mass of Al ₂ O ₃ . 2. Broadband probe according to claim 1, characterized in that the edge plate (3) is U-shaped. 3. Broadband probe according to claim 1, characterized in that the partially stabilized tetragonal ZrO ₂ used to produce the plates contains 4-5 mol% of Y ₂ O ₃ . 4. Broadband probe according to claim 1, characterized in that the fully stabilized ZrO ₂ 8 mol% used for the production of the plates contains Y ₂ O ₃ . 5. Broadband probe according to one of claims 1 to 4, characterized in that solid electrolyte plate (4) and cover plate (10) have Pt electrodes and a Pt area sufficient to form a 3-phase boundary. 6. Broadband probe according to one of claims 1 to 5, characterized in that the diffusion gap, for example by a Ring channel (9) in the insulating plate (7) or a recess (17) in the base plate (16) is formed.