DE19937163A1 - Screen-printing paste used to make flat ceramic components for lambda sensors used especially in vehicles, includes magnesium titanate or its mixture with spinel, forsterite or magnesia - Google Patents

Abstract

The mineral content is MgTiO3 or its mixture with spinel, forsterite or magnesia. An Independent claim is included for the method of making the sensor element. A heater (3) on a carrier sheet (1) based on ZrO2 is embedded in an insulating mass (4). The green assembly is sintered, the novel feature being that the insulating masses (2, 4) contain a mineral proportion based on a magnesium-containing oxide.

Description

State of the art

The invention relates to a screen printing paste for the Manufacture of planar ceramic elements, a ceramic sensor element and a method for the production.

Ceramic sensor elements with a carrier based on zirconium oxide and a heating device that is electrically insulated and airtight against the carrier, in particular for planar lambda sensors, are produced in the thick-film process by screen printing functional layers onto unsintered ZrO ₂ foils and subsequent sintering. Since the zirconium oxide has a non-negligible electrical conductivity when using such sensors, porous sintering Al ₂ O ₃ -based screen printing pastes are used as insulation layers between the heating device and the neighboring ZrO ₂ foils, in which the heating device is embedded .

Due to the open-pore structure of the sintered insulation layers, a side sealing frame made of ZrO ₂ around the heating device and its embedding is required in order to obtain an all-round gas-tight surface of the sensor element. This is the only way to avoid falsification of the reference air and a corrosive attack on the heating system caused by exhaust gas components.

The sintering behavior of the materials currently used to embed the heating device differs significantly from that of the neighboring ZrO ₂ foils. Since the sintering shrinkage of these materials begins at higher temperatures than that of the ZrO ₂ foils, bending stresses occur in the sensor element. A curvature of the sensor element can only be avoided by a sufficient weight. In addition, the lower coefficient of thermal expansion of Al ₂ O ₃ of 8.3 × 10 ^-6 K ^-1 versus over 10.0 × 10 ^-6 K ^-1 with ZrO ₂ leads to permanent tensile stresses in the ZrO when cooling from the sintering temperature to room temperature _2nd In order to prevent these tensile stresses from cracking in the course of the use of the sensor, compositions are used as embedding material which sinter to a po porous structure with a low modulus of elasticity, which exerts only a small tensile load on the surrounding ZrO ₂ layers.

In DE 44 39 883 forsterite was used as a material for the electrical insulation of the heater beaten.  

The material (ASB) currently used for embedding the heating device is produced in a complex process from the components Al ₂ O ₃ , SiO ₂ and BaO or BaCO ₃ . The properties of the pastes obtained therefrom fluctuate greatly. Quality problems in sensor element production are partly due to the poor reproducibility of the paste properties.

Advantages of the invention

The present invention firstly proposes a screen printing paste for the production of planar ceramic elements which sinters at the temperatures customary for sintering ZrO ₂ to a gas-tight body with an expansion coefficient adapted to the thermal expansion coefficient of ZrO ₂ . This is achieved in that the mineral portion of the paste is essentially MgTiO ₃ or a mixture of MgTiO ₃ with one or more other magnesium-containing oxides, in particular MgO, spinel or forsterite.

A method for producing a sensor element is also proposed, which allows the production of sensor elements with a carrier film based on ZrO ₂ with a simpler structure. Namely, in a method in which a blank of the sensor element is first produced by embedding a heating device on a carrier film based on ZrO ₂ in an insulating mass and then sintering the blank, an insulating mass with a mineral part based on MgTiO ₃ or a mixture of MgTiO ₃ with one or more other magnesi-containing oxides is used, a gas-tight sintered body can be obtained by sintering at the temperatures customary for the sintering of ZrO ₂ from the insulating material, so that the conventional side sealing frame is not required can, which are required when using insulating materials based on Al ₂ O ₃ due to their porosity after sintering.

A sintering behavior that is very similar to that of ZrO ₂ can be achieved with a mineral portion in the form of magnesium titanate powder with an average grain size of 1.5-2.5 µm.

It is preferably the insulating compound around the screen printing paste described above.

For the production of a sensor, a first layer of this paste is preferably printed on the carrier film based on ZrO ₂ , the heating device is printed on it and finally a second layer of the paste is printed on the heating device in order to embed the latter. Covering the paste with another film made of ZrO ₂ material is not absolutely necessary, since the paste sinters gas-tight and thus protects the heating device embedded in it against oxidation.

According to a further aspect of the invention, a sensor element with a carrier based on ZrO ₂ and a heating device which is insulated and hermetically sealed against the carrier is proposed, in which the heating device in a body made of magnesium titanate or of a mixture of MgTiO ₃ with Mg containing oxides such as MgO, spinel or forsterite is embedded, which has essentially the same coefficient of thermal expansion as the ZrO _{2 of} the carrier.

It is preferably the sensor element around a lambda sensor element.

Further features and advantages of the invention result from the following description of Ausfüh tion examples with reference to the accompanying Figu ren.

characters

Fig. 1 shows a schematic Explo sionsdarstellung the structure of a lambda sensor element;

Fig. 2 illustrates the shrinkage behavior during sintering of magnesium titanate and ZrO ₂ as a function of temperature;

Fig. 3 shows the temperature dependence of the coefficient of thermal expansion of magnesium titanate and ZrO ₂ ; and

Fig. 4 shows the temperature dependence of the electrical conductivity of magnesium titanate, alumina, and ZrO _2.

Description of the embodiments

Fig. 1 shows an exploded view of the structure and manufacture of a lambda sensor with a sensor element with a sealing heater without a sealing frame. On a lower foil called Heizerfo lie 1 based on ZrO ₂ , a lower insulation layer is applied by screen printing, which has a mineral content in the form of magnesium titanate powder with a mean grain size of 1.5-2.0 µm. The screen printing paste has the following composition:

• Magnesium titanate powder: 20 to 70% by weight,
• Solvent: 20 to 70% by weight,
• Binder: 1 to 15% by weight,
• Plasticizer: 1 to 15% by weight.

A typical composition is:

• 40% by weight of magnesium titanate powder,
• 50% by weight of organic solvents,
• 70% by weight of organic binder,
• - 3% by weight of organic plasticizer.

Examples of solvents are butyl carbitol or ethylhexanol in question, as a binder was Po lyvinylbutyral used; are typical plasticizers Dibutyl phthalate or dioctyl phthalate.

A screen printing paste with the above-mentioned composition is produced, for example, by homogenizing in suitable mixing units such as, for example, a ball mill, mortar mill, a three-roll mill, an attritor or dissolver. The paste is then printed onto the unsintered ceramic heater sheet 1 .

The electrical heating device 3 in the form of a meandering conductor track and supply lines 14 for the power supply of the heating device 3 are also applied to this layer by screen printing. A second insulation layer 4 with the same composition as the insulation layer 1 is printed over it. The heater 3 is thus embedded all around in the insulation layers 2 and 4 . The leads 14 are through contacts 12 in the lower insulation layer 2 and the heater foil 1 with zer foil 1 attached to the outer surface of the heater 1 connected contacts 11 connected. Since these contacts 11 are attached to the opposite of the heater 3 and therefore relatively cool end of the heater sheet 1 , there is no danger that they will oxidize ambient air of the sensor in the environment. Alternatively, the feed lines 14 could be led directly from the embedding between the two insulation layers 2 , 4 in order to supply the heating device 3 with current. Another alternative is a version with a continuous external heating device, which is only covered by a sealing-sintered insulation layer, the connection contacts remaining free.

On the second insulation layer 4 , a second foil made of zirconium oxide, referred to as the reference air duct foil 5 , is arranged. A reference Luftka channel 6 extends over almost the entire length of the film 5 from which the contacts 11 carry the "cold" end of the sensor into the "hot" area above the heater 3rd

On the reference air duct 5, in turn, a sensor film 8 is arranged, which carries an inner or an outer electrode 7 or 9 in the area above the heating device 3 on its two surfaces. The outer electrode 9 is shielded from the atmosphere surrounding the sensor by a porous protective layer 10 .

The sintering behavior of the magnesium titanate powder of the insulation layers 2 and 4 is shown in FIG. 2 together with that of the ZrO ₂ films 1 and 5, respectively. The temperature in ° C is plotted on the ordinate of the diagram and the shrinkage in percent on the abscissa. For the measurement of the shrinkage behavior of magnesium titanate powder axially pressed samples were used from the pure powder under 500 bar, which were heated with a heating rate of 100 K per hour. The sintering of the magnesium titanate at about 1000 ° C is on the same level as that of the ZrO ₂ films with the addition of SiO ₂ and Al ₂ O ₃ conventionally used for the production of planar sensor elements.

The shrinkage increases above 1000 ° C and reaches a value of approx. 12% at 1300 ° C, as can be seen in curve 20 .

For a first sample, the heating process was stopped at 1300 ° C and the temperature was maintained for two hours. Curve 20 A shows the further sintering behavior of this sample: the shrinkage continued to a final rate of approx. 15%, the density of the sintered body obtained was 3.71 g / cm ³ . In two further samples, the heating was discontinued at 1350 ° C. and 1375 ° C., and the temperature then reached was maintained for two hours. As the associated curves 20 B and 20 C show, the sintering behavior of these samples is very similar to the former. In both cases, sintered bodies with a density of 3.72 g / cm ^{3 were} obtained.

The theoretical density of magnesium titanate is 3.88 g / cm ³ . The sintered bodies thus reached 95.9% of this theoretical density. The sintered material therefore has a closed porosity and is therefore gas-tight.

For comparison, curve 21 shows the shrinkage behavior of a ZrO ₂ film used in the conventional manner for producing a sensor element. In addition to the above-mentioned additions of Al ₂ O ₃ and SiO ₂ and yttrium stabilization in the unsintered state, this film contains an organic part which binds the mineral parts and makes the film manageable. In order to give this organic fraction time to burn and outgas, the ZrO ₂ film was first heated to approximately 800 ° C. at a heating rate of 30 K per hour. A shrinkage of approx. 2% of the ZrO ₂ film in the temperature range below 1000 ° C is due to the combustion of this organic component. Above 800 ° C, the film was heated to a final temperature of 1380 ° C at a heating rate of 90 K per hour and held at this temperature for five hours. A final density of the ZrO ₂ film of 5.86 g / cm ^{3 was} achieved.

As with magnesium titanate, the sintering process begins at approx. 1000 ° C and is essentially completed at just under 1400 ° C. The sintering behavior of ZrO ₂ and magnesium titanate is not only the same in terms of shrinkage rates, but also in terms of the temperatures of the sintering process. If one were to measure the shrinkage behavior of a screen printing paste containing this material as a mineral component instead of a sample of pure magnesium titanate, an even greater similarity of the shrinkage curves would be expected than shown in curves 20 , 21 , because in such a case would Even with the magnesium titanate paste, the combustion of the organic portion leads to a shrinkage below 1000 ° C in a similar order of magnitude as with the ZrO ₂ film.

Fig. 3 shows the course of the thermal expansion coefficient of sintered magnesium titanate and sintered ZrO ₂ film depending on the temperature. The temperature in ° C is plotted on the ordina te, the thermal expansion coefficient in 10 ^-6 K ^-1 on the abscissa of the diagram. Curve 30 shows the behavior of magnesium titanate, curve 31 that of ZrO ₂ film. The coefficient of expansion of the sintered magnesium titanate body is 10.4 × 10.6 K ^-1 at 1100 ° C and corresponds to that of ZrO ₂ . Only slight deviations occur in the lower temperature range. Therefore, when cooling composite materials made of ZrO ₂ and MgTiO ₃ , just like a sensor element of the type described above, there are only low permanent tensions due to thermal mismatch.

Due to the thermal properties of ZrO ₂ and MgTiO _{3 shown} with reference to FIGS . 2 and 3, the use of magnesium titanate leads to a considerably lower probability of rejects in the production of sensor elements. Since the finished sensor elements are poor in internal voltages, there is also the likelihood that temperature changes during the operation of the sensor elements lead to the formation of macroscopic cracks and to the destruction of the sensor elements. The sensitivity to temperature shocks is thus reduced.

Fig. 4 shows the temperature dependence of the electrical conductivity of a sintered body made of magnesium titanate and, in comparison, that of sintered Al ₂ O ₃ and ZrO ₂ . The reciprocal of the temperature in 1000 K ^-1 and the temperature in K are plotted on the ordinate of the diagram below; on the abscissa the conductivity is given in S / cm. The conductivity was measured during the heating and cooling of a sample. Curve 40 shows the behavior of a ZrO ₂ film. Above a temperature of approx. 400 K, an increase in conductivity can be seen. In the temperature range above 1000 K that is of interest for the use of a ceramic sensor element as a lambda sensor, the conductivity of the ZrO _{2 is} over 1 mS / cm. It is therefore necessary to electrically insulate the heating device of such a sensor element against a ZrO ₂ carrier film. Curve 41 shows the conductivity of an aluminum oxide conventionally used therefor. It only rises continuously at approx. 650 K and reaches a conductivity of approx. 10 ^-6 to 10 ^-5 S / cm in the temperature range 1000 to 1200 K.

Curve 42 shows the conductivity of magnesium titanate. Like aluminum oxide, it begins to increase continuously from a temperature of approx. 650 K. The increase is somewhat steeper than with aluminum oxide and reaches a conductivity of approx. 1 × 10 ^-4 S / cm at 1200 K. In the temperature range of interest, the conductivity is approximately two orders of magnitude below the value of ZrO ₂ . Magnesium titanate therefore has sufficient insulation properties even at high temperatures.

Although the invention has been described above with regard to the use of magnesium titanate as a mineral component of the screen printing paste, or the insulation layer of a sensor element produced using the screen printing paste, it is expected, however, that other magnesium-containing oxides, in particular mixtures of MgTiO _{3 have} similar advantageous properties with forsterite, spinel or MgO.

Claims (13)

1. Screen printing paste for the production of planar ceramic elements, containing a mineral component, solvents and auxiliaries, characterized in that the mineral component is essentially MgTiO ₃ or a mixture of MgTiO ₃ with spinel, forsterite or MgO. 2. Paste according to claim 1, characterized in that they have between 20 and 70 wt .-% mineral part, between 20 and 70% solvent, between 1 and 15% by weight of binder and between 1 and 15% by weight Plasticizer includes. 3. Paste according to claim 2, characterized in that they have about 40 wt .-% mineral content, about 50 % By weight of solvent, approx. 7% by weight of binder and approx. 3 % By weight plasticizer. 4. A method for producing a sensor element in which a blank of the sensor element is produced by a heating device ( 3 ) on a carrier foil ( 1 ) based on ZrO _{2 is} embedded in an insulating mass ( 4 ), and the blank is sintered , characterized in that the insulating compound ( 2 , 4 ) has a mineral content based on a magnesium-containing oxide. 5. The method according to claim 4, characterized in that the magnesium-containing oxide is MgTiO ₃ or a mixture of MgTiO ₃ with MgO, spinel or forsterite. 6. The method according to claim 5, characterized in that the mineral content is MgTiO ₃ powder with an average grain size of about 1.5-2.5 microns. 7. The method according to any one of claims 4 to 6, there characterized in that the sintering at a Temperature between 1000 and 1400 ° C, in particular carried out at a maximum temperature of 1385 ° C becomes. 8. The method according to any one of claims 4 to 7, there characterized in that the insulating compound has a Pa is according to one of claims 1 to 4. 9. The method according to claim 8, characterized in that a first layer ( 2 ) of the paste is printed on the carrier film ( 1 ), the heating device ( 3 ) is printed on the first layer ( 2 ) and a second layer ( 4th ) the paste is printed on the heating device ( 3 ) to embed the latter. 10. Sensor element with a support ( 1 ) on the basis of ZrO ₂ and an isolated and airtight enclosed heating device ( 3 ) characterized in that the heating device ( 3 ) in a body ( 2 , 4 ) based on a magnesi containing Oxide is embedded. 11. Sensor element according to claim 10, characterized in that the magnesium-containing oxide is MgTiO ₃ or a mixture of MgTiO ₃ with MgO, spinel or For sterite. 12. Sensor element according to claim 11, characterized in that the body ( 2 , 4 ) is sintered from MgTiO ₃ powder with an average grain size of about 1.5-2.5 microns. 13. Sensor element according to one of claims 10 to 12, characterized in that it is a lambda Sensor element is.