EP0435999B1 - Temperature sensor and process for producing the same - Google Patents

Abstract

A temperature sensor with platinum as thermally sensitive material is disclosed, as well as a process for producing the same. The object of the invention is to obtain a miniaturized temperature sensor as small as possible which can be used for temperatures extending from above 600 degrees to above 1000 degrees. For that purpose, the oxide ceramic layer (7) contains finely distributed metallic platinum. The sensor is produced by mixing the platinum powder, the oxide materials and the binder, (19) then by tempering the layer (7) composed of said mixture together with the substrate (19), after the layer (7) has been applied on the substrate (19).

Description

The invention relates to a temperature sensor according to the preamble of the main claim.

Temperate sensors with platinum made of a temperature-sensitive element are known, which are manufactured using thin-film technology, in which platinum is thus diffused onto a carrier substrate in a few atomic layers. If a corresponding geometric structure, such as a meandering shape, is maintained, a sufficiently high basic resistance can be achieved with sufficiently thin layers by means of a few atomic layers, which for such a temperature sensor must be in the range of 100 ohms. These thin-film sensors can only be used at lower temperatures in the range up to 400 degrees C, in any case below 600 degrees C, since platinum evaporates at higher temperatures and, due to the thickness of the layer of only a few atomic layers, this alone makes it non-negligible The resistance is changed so that reproducible measurements are no longer possible.

Platinum wires have also been used as temperature sensors. In order to achieve the sufficiently large basic resistance, the wire had to have a considerable length, which, even in the case of winding in coil form, only led to a sensor with considerable external dimensions, which cannot be used in many areas where miniaturization is important. Platinum-containing thick-film pastes are also known which contain organic binders and solvents as further constituents. These are used as thick film heating elements, where they reach their sufficient resistance due to the length of the heating element and are used for heating the usual pressure point temperatures. Apart from the low specific resistance, which would also only lead to very large sensor elements, these could not be used for temperatures higher than 600 degrees C, since reproducibility would no longer be ensured here. In both cases, moreover, platinum becomes red-hot from approx. 800 degrees C and can then still be used as a radiator, but no longer as a temperature sensor.

It has been found that no miniaturized temperature sensors are known for high temperature applications. Thermocouples that work with thermal voltages are used. The disadvantage is that a defined ambient temperature must be used as the reference temperature or a microprocessor. Such temperature sensors are expensive.

A temperature sensor is known (GB-A 2 120 453) which consists of a ceramic substrate on which a temperature-sensitive film layer of an electrical resistance element is applied. A commercially available organometallic paste is used which does not contain any ceramic or purely metallic components. This can consist of platinum or a platinum-containing ceramic material. No further information on the composition is disclosed.

There is also known a generic resistance thermometer according to the preamble of the main claim (GB-A 2 006 521), the temperature-sensitive element of which consists of platinum-containing ceramic bodies. The platinum content is over 90% and the sintering takes place over a period of two hours at a temperature of 1200 ° C. As a result, however, a high-temperature stable thermometer cannot be obtained.

The invention is therefore based on the object of providing a sensitive, time-stable, high-temperature sensor in a miniaturized design which can be used in a variety of ways.

According to the invention, the stated object is achieved according to the invention in a generic temperature sensor according to the preamble of the main claim by its characterizing features. To produce such a temperature sensor, a method is proposed according to the invention, which is characterized in that platinum powder, oxides and binders are mixed with one another and, after the layer has been applied to the carrier substrate, are heat-treated with the latter.

The temperature-sensitive layer of the temperature sensor according to the invention has metallic platinum between 60 and 90% by weight and is preferably characterized in that it contains metallic platinum in a proportion of approximately 70 to 85% by weight. A mixture of silicon, aluminum and alkaline earth oxide, in particular calcium oxide, is used as the oxide component, the aluminum oxide component resulting from the fact that the carrier substrate is generally aluminum oxide. In the event that the carrier substrate consists of a different oxide, the aluminum oxide could also be replaced by the material of the corresponding carrier substrate. Due to the tempering, silicon oxide produces quartz or the glass-like character and forms an inert material that is particularly suitable for the desired high-temperature applications. Calcium oxide is the preferred alkaline earth oxide; strontium and barium oxides can also be used instead, but calcium oxide has proven to be more stable. The oxide mixture forms a eutectic, the melting point of which can be adjusted and in particular reduced by the addition of the alkaline earth oxide, while a mixture of aluminum oxide and silicon oxide gives a relatively high melting point which is above the evaporation point of platinum, so that no heating up to this point could take place . By adding calcium oxide, the melting point of the eutectic mixture is reduced below the evaporation point of platinum, so that annealing can take place up to the desired melting point of the oxide mixture, at which the desired quartz-like or glass-like compact consistency of the material is achieved. Accordingly, the temperature sensitive Layer of the temperature sensor preferably has a composition such that silicon oxide is present in the oxide mixture in a range from 40 to 55% by weight, aluminum oxide to 25 to 40% by weight and the remainder is alkaline earth oxide, while silicon oxide with 45 to 50% by weight is extremely preferred. %, Aluminum oxide with 30 to 35% by weight, rest of alkaline earth oxide is given, in particular the oxide mixture containing 18 to 20% by weight of alkaline earth oxide and the rest of silicon oxide and aluminum oxide. In order to achieve the greatest possible reduction in the melting point of the eutectic oxide mixture, the invention accordingly provides that the temperature-sensitive layer is baked on the carrier substrate, the temperature-sensitive layer in particular having a compact, glass-like morphology. An ideal oxide mixture is explained in the following description of the figures. The specific electrical resistance is determined by the ratio of platinum and oxide content in the temperature-sensitive layer. Care must be taken to ensure that the proportion of platinum is not reduced to such an extent that the required conductivity bridges are completely interrupted or are easily interrupted during use. In this respect, it has been found to be advantageous that platinum should have a proportion of 80% by weight in the temperature-sensitive layer, based on the total mixture itself, between 60 and 80% by weight. Accordingly, the process uses the oxide mixture with a share of 14 to 20 wt .-% of the total mixture of platinum paste, oil and thinner. In a preferred embodiment, it is provided that platinum paste with 65 to 70% by weight, oil and thinner each with 5 to 10% by weight and the rest of oxide are mixed with one another, the platinum content of the paste itself preferably being 75% by weight. A preferred specific overall composition of the thick-film printing compound used according to the invention can also be found in the description of the figures.

While the maximum temperature of the tempering should remain below the evaporation temperature of platinum and is preferably below 1400 degrees, a preferred embodiment provides a maximum temperature of 1300 to 1350 degrees.

In a further development it is provided that a holding time for the complete combustion of organic binder portions of the layer is maintained in the range from 300 to 400 degrees C. The holding time serves to generate the desired compact, glass-like consistency of the temperature-sensitive layer of the temperature sensor to be created. The maximum holding time is not critical per se, but must not be unduly excessive, since in addition to the desired morphological changes leading to the compact, glass-like consistency, changes in the platinum base structure due to sintering effects, which lead to undesired larger structures or plaster formation, and possibly also to oxidation and lead to a breakdown of the platinum surfaces. A holding time of 20 to 40 minutes is therefore preferred, a time of 25 minutes having proven to be an optimal value.

The steady, not too steep temperature rise and fall is necessary in view of the fact that the temperature-sensitive layer must not be exposed to a temperature jump during the tempering process, since this could lead to damage such as embrittlement and cracks. Accordingly, a temperature control with a temperature coefficient of 10 to 15 degrees C per minute and in particular of 13 degrees C per minute for the temperature rise and drop, the latter in particular above about 1100 degrees C, has been found. While this relates to the temperature control on the heating element of the sintering furnace in terms of the drop, the design of the furnace can cause an overall slower drop in temperature.

If a platinum paste containing in particular organic binders - which are generally cellulose derivatives - is used, it is advantageous in the region of the temperature rise at 300 to 400 degrees C, especially at 350 degrees C, also to provide a holding time during which the temperature is held at a fixed value over a predetermined time. While the maximum duration of the holding time is ultimately only limited by economic requirements, the holding time should not be too short, in particular not less than five minutes, in order to achieve a perfect result. About ten minutes turned out to be the ideal value. If such a holding time is used, the tempered layer has a typical light quartz / ceramic color, while if the holding time is too short in the area mentioned, the color darkens until it becomes black. This is because the orange binder burns only slowly and does not completely burn to CO₂ if the holding time is not sufficient in the temperature range mentioned, but rather carbon portions remain, which can also have a negative effect on the temperature-sensitive properties of the layer.

Overall, the invention creates a miniaturized temperature sensor, which can preferably be used at temperatures of over 600 degrees C to 1200 degrees C. The temperature sensor according to the invention is inexpensive to manufacture and can in particular also be applied simultaneously and together with other functional elements, such as oxygen sensors, which are produced using the same technology, and heat conductors can be applied to a common substrate. Thus, a preferred embodiment provides that an oxygen sensor and a heating conductor regulated by the temperature-sensitive layer are applied to the carrier substrate, and in a further development that the heating conductor is applied to the surface of the carrier substrate which carries the oxygen sensor and the temperature-sensitive layer. In particular, the manufacture of the sensor according to the invention is cheaper than thin-film technology, apart from the fact that no sensors of this type that can withstand high temperatures can be produced as a result. No vacuum and no complex equipment are necessary. Furthermore, the effect of the temperature measurement result is also not to complicated specifications, such as environmental sensors, which are required for measurements using thermal voltage or additional electronics. Rather, the sensor output can be used directly to control, for example, a heating conductor.

The above combination is used in particular for raw oxygen measurement, for example in gas power plants or in process control technology in the chemical industry, if the residual oxygen content is to be measured there with regard to inerting. The lambda value measurement takes place on the basis of a solid-state effect with reduction or oxidation depending on the oxygen content of the ambient gas, this solid-state effect only occurring at higher temperatures, in particular temperatures above 600 degrees C., so that the temperature sensor must be heated to this temperature and to the desired one Specification temperature must be kept with high accuracy, for which the temperature sensor according to the invention can be used in an ideal manner. Other areas of application concern high-temperature furnaces, sintering furnaces etc.

Figur 1

Eine bevorzugte Ausgestaltung eines erfindungsgemäßen Temperatursensors;

Figur 2

eine bevorzugte Temperaturführung beim Tempervorgang zur Herstellung des Temperatursensors.

Further advantages and features of the invention result from the claims and from the following description, in which an embodiment of the invention is explained in detail with reference to the drawing. It shows:

Figure 1

A preferred embodiment of a temperature sensor according to the invention;

Figure 2

a preferred temperature control during the tempering process for producing the temperature sensor.

Oxygen sensors for lambda measurement, such as gas power plants, process technology, etc., show their highest sensitivity, which is based on an oxidation-reduction-solid-state effect corresponding to the available oxygen, at higher temperatures. They must therefore be heated to higher temperatures and, since the effect changes depending on the temperature, stabilized at a predetermined temperature. For this purpose, the oxygen or gas sensor 2, which is known per se, can be applied to a substrate or carrier 1, such as made of aluminum oxide. A heating conductor 6, which can be, for example, a heating conductor on a ceramic basis, is applied to the surface 4 of the carrier 1 opposite the oxygen sensor 2. Furthermore, a temperature sensor 7 is applied near the oxygen sensor 2 on the surface 3 of the carrier 1 in the manner described below. The temperature sensor 7 is guided in a meandering shape and has, for example, an overall length of 10 mm, a width of 3 mm, a total "wire length" of 60 to 70 mm and a layer thickness of 10 to 15 micrometers and a width of 250 micrometers in the composition specified below on.

The temperature sensor 7 consists of a ceramic - which is preferably largely "glazed" due to the tempering process - of oxide and pure metal platinum dispersed in this with a proportion of 80% by weight. According to a preferred embodiment, the oxide composition is 50% by weight of silicon oxide, 30% by weight of aluminum oxide and 20% by weight of calcium oxide. The basic resistance of the temperature sensor 7 described in this way is approximately 100 ohms.

The temperature sensor 7 is produced on the carrier substrate 1 in the following way:

First, platinum powder and oxide are mixed with the desired final proportions of 80% by weight and 20% by weight. A paste is then produced from 65% by weight of platinum and oxide powder and 35% by weight of vehicle. The vehicle consists of 70% by weight of an organic binder, such as methyl cellulose, and 30% by weight of an organic solvent, such as dibutyl carbitol acetate. The paste obtained in this way is then printed on the carrier substrate 1 made of aluminum oxide in the desired geometric shape, such as the meandering shape shown, using screen printing and thus thick film technology.

An annealing is then carried out, with the support 1 and the printed-on temperature sensor substrate being heated to about 350 degrees C. in a tempering furnace, starting from ambient temperature (20 degrees) with a differential temperature increase of about 13 degrees C. per minute. Solvents, thinners and oil evaporate above their evaporation temperature. From about 100 degrees C, the previously viscous printing mass is an almost solid mass, since the liquid components are largely burned. Furthermore, the organic binder, which is a cellulose derivative, begins to burn. Since the organic binder burns slowly, the temperature is kept constant at about 350 degrees for about 10 minutes to allow complete combustion (conversion to CO₂) of the organic binder. It was found that the oxide ceramic obtained was black or dark due to an incompletely burned binder, with no or insufficient holding time, while if a sufficient holding time was observed in the temperature range mentioned, the ceramic ultimately obtained had the typical light color due to the complete combustion of the binder. Binder that is not completely burned could also impair the properties of the temperature sensor. After the holding time shown in FIG. 2 at a temperature of 350 ° C., there is a further temperature increase with the same temperature coefficient up to the desired final or maximum stoving temperature of approx. 1330 ° C. It has been shown that the temperature rise is a critical variable is. With a steeper temperature rise, cracks occur in the sensor layer. A flat rise in temperature is quite possible, but this means longer production times and thus higher production costs and higher costs. In this respect, the specified temperature control provides an optimization while ensuring a flawless result represents.

The specified baking temperature of 1330 degrees C is maintained for a certain time, which was 25 minutes in the illustrated embodiment. This is necessary in order to cause the layer to flow into one another and thus to change the morphology (while maintaining the structure) and overall to achieve a glass-like, more compact layer which ensures uniform conductivity. It should be noted that the baking temperature should not be maintained too long, since internal structural changes then take place, in particular platinum bridges are apparently broken and the continuous electrical contact is damaged, be it due to typical sintering effects in the form of larger structures or plaster formation , be it due to oxidation of plate particles. While with regard to the desired compact glass-like consistency the mentioned holding time at the stoving temperature can hardly be shortened, a certain lengthening is not critical since the above-mentioned disadvantageous effects only occur when the stoving temperature is excessively long. In this respect, too, an optimization is carried out in such a way that the holding time of the baking temperature was chosen to be as short as possible, it being ensured that the desired compact, glass-like structure is achieved. The temperature is then reduced using the same temperature coefficient on the heating element, ie the same temperature control. Due to the furnace's own cooling behavior, the temperature in the furnace cools more slowly, as shown. It is essential to cool the temperature with the temperature coefficient mentioned to approximately 1100 ° C., which should not be chosen more, since otherwise the structure obtained could also be damaged by bus.

Overall, according to the invention there is a temperature sensor with a sufficiently high basic resistance in the specified range, which is resistant to high temperatures and in particular with Temperatures of over 600 to well over 1000 degrees C can be used for temperature measurement and thus for temperature control of the heater 6 in the exemplary embodiment shown in FIG. 1.

Claims (18)

1. Temperature sensor with a temperature-sensitive layer deposited on a carrier substrate and containing platinum, said layer containing metallic platinum finely distributed in oxide ceramic, in which the layer contains metallic platinum in a content of 60 to 90% by wt, and the oxidic portion of the temperature-sensitive layer (7) is an oxidic mixture of silicon, aluminium and alkaline earth oxides, in particular calcium oxide, characterised in that silicon oxide is present in the oxidic mixture in a range of 40 to 55% by wt, aluminium oxide in a range of 25 to 40% by wt and the balance is alkaline earth oxide.
2. Sensor according to claim 1, characterised in that the oxidic mixture contains 18 to 20% alkaline earth oxide and the balance is silicon oxide and aluminium oxide.
3. Sensor according to any one of the preceding claims, characterised in that the temperature-sensitive layer (7) is burnt onto the carrier substrate (1).
4. Sensor according to any one of the preceding claims, characterised in that the temperature-sensitive layer (7) exhibits a compact, glass-like morphology.
5. Sensor according to any one of the preceding claims, characterised in that on the carrier substrate (1) there is deposited an oxygen sensor (2) together with a heating conductor (6) which is controlled by the temperature-sensitive layer (7).
6. Sensor according to claim 5, characterised in that the heating conductor (6) is deposited on the surface (4) of the carrier substrate (1) facing away from the surface (3) bearing the oxygen sensor (2) and the temperature-sensitive layer (7).
7. Method for manufacturing a temperature sensor according to any one of claims 1 to 6, in which a platinum-containing layer is deposited on a carrier substrate, characterised in that platinum powder, oxides and binder are mixed with one another and after the application of the layer to the carrier substrate are tempered therewith.
8. Method according to claim 7, characterised in that at least one oil is used as binder.
9. Method according to claim 7 or 8, characterised in that platinum-powder mixed with organic binder and solvent is used as a paste.
10. Method according to any one of claims 7 to 9, characterised in that a thinner is also incorporated for adjusting the consistency.
11. Method according to any one of claims 7 to 10, characterised in that the oxidic portion is used as a silicon, aluminium or alkaline earth oxide mixture.
12. Method according to claim 11, characterised in that an oxidic mixture containing silicon oxide in a range of 45 to 50% by wt, aluminium oxide 30 to 35% by wt and the balance alkaline earth oxide, in particular calcium oxide, (calculated on the total oxide weight) is used.
13. Method according to claim 11 or 12, characterised in that an oxidic mixture with an oxidic content of 18 to 20% alkaline earth oxide and the balance silicon oxide and aluminium oxide (calculated on the total oxide weight) is used.
14. Method according to any one of claims 7 to 13, characterised in that oxide in a proportion of 14 to 20% by wt of the total mixture weight is used.
15. Method according to any one of claims 7 to 14, characterised in that platinum paste in 65 to 70% by wt, oil and thinner in respectively 5 to 10% by wt and the balance oxide are mixed with one another.
16. Method according to any one of claims 7 to 15, characterised in that the tempering takes place at a temperature of between 1300 and 1350 °C.
17. Method according to any one of claims 7 to 16, characterised in that the heating temperature control curve ascends uniformly in the main and descends in the same manner with a final holding time at the maximum tempering temperature.
18. Method according to any one of claims 7 to 17, characterised in that in the range of 300 to 400 °C a holding time is observed for the complete incineration of organic binder components of the layer.

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