Patent Description:
Sensor assemblies for resistance temperature sensor elements are known in the art. Examples can be found in <CIT> and <CIT>. These sensor assemblies comprise measuring structures, in particular resistive elements. Such a measuring structure is usually made from platinum. A change in temperature induces a change in the electric resistance of the measuring structure. This electric resistance can be measured and the corresponding temperature of the element can then be calculated. The measuring structure is usually supported by a substrate.

Known sensor assemblies for resistance temperature sensor elements are critical with respect to their structural integrity over their life time. In particular, a large number of temperature changes may reduce the structural integrity of the assembly and lead to failure of the assembly and of the resistance thermometer.

It is therefore the object of the invention to provide a sensor assembly for a resistance temperature sensor element that maintains its structural integrity over a large number of cycles of temperature changes.

For the sensor assembly according to the invention, this object is achieved by a sensor assembly for a resistance temperature sensor element, the sensor assembly comprising a substrate and a measuring structure disposed on the substrate, wherein the substrate comprises at least one first material, the first material being at least one of aluminum oxide, spinel (magnesium aluminate) and yttrium-aluminum-garnet, at least one stabilized second material, the stabilized second material being at least one of stabilized zirconium dioxide and stabilized hafnium dioxide, the stabilized second material being stabilized by containing an oxide of an element having a valence different from four, wherein a coefficient of thermal expansion of the substrate deviates by less than <NUM> % from a coefficient of thermal expansion of the measuring structure and characterized in that the stabilized second material is stabilized by containing an oxide of an element with valence three or five.

For the resistance temperature sensor element according to the invention, the object is achieved in that the resistance thermometer comprises at least one sensor assembly according to the invention.

Due to the coefficients of thermal expansion of the substrate and the measuring structure, which differ not more than <NUM>% from each other, the coefficients of thermal expansion are regarded as being matched.

This matching is important, since in case of a mismatch of the CTEs, a change in temperature may induce different volume changes in the substrate and in the measuring structure. This results in stress effects on the measuring structure. This can cause a shift in resistance and temperature coefficient of resistance. Further, the measuring structure may undergo geometric changes. One effect can be hysteresis in resistance and temperature coefficient of resistance with temperature cycling. Overall, the bond between the measuring structure and the substrate may be damaged. It is also possible that the measuring structure gets destroyed.

Due to the stabilized second material, the invention allows to achieve a good matching of the coefficients of thermal expansion even across a wide temperature range. This is explained in the following.

The measuring structure is usually made from platinum. However, it is not limited to platinum. Platinum has a CTE of <NUM> ppm/K. It is known that zirconium dioxide has an average CTE of <NUM> ppm/K. With aluminum oxide having a CTE of about <NUM> ppm/K, one may assume that mixing these materials is applicable to reach the CTE of platinum.

However, pure zirconium dioxide has a CTE of <NUM> ppm/K only in some directions of its crystal structure at room temperature. The average CTE of pure zirconium dioxide at room temperature is only about <NUM> ppm/K.

Hence, mixing pure zirconium dioxide with aluminum oxide may not result in a substrate having a CTE that is matched with that of platinum. The aforementioned problem also exists for pure hafnium dioxide.

At temperatures above <NUM>, pure zirconium dioxide has an average CTE of <NUM> ppm/K, because the material then has an overall tetragonal crystal structure. Above <NUM>, the crystal structure becomes cubic. However, for measurements below <NUM>, this is not useful.

The inventive solution overcomes this problem by using stabilized zirconium dioxide or stabilized hafnium dioxide.

To get stabilized materials, pure zirconium dioxide or pure hafnium dioxide is mixed with a stabilizing material. This stabilizing material is preferably an oxide of an element having a valence that differs from the valence of zirconium. The valence of zirconium is <NUM>.

Hence, an element having a valence being <NUM> or <NUM> is used for the stabilizing material.

Mixing pure zirconium dioxide with such an oxide will result in a stabilized material having a stabilized cubic and/or tetragonal crystal structure even at room temperature. With this structure, the stabilized material has a homogeneous CTE. The same is also valid for hafnium dioxide.

To summarize, the sensor assembly according to the invention allows for a fine adjusted matching of the coefficients of thermal expansion of the measuring structure and the substrate. Thereby, a reliable sensor assembly is achieved.

The aforementioned adjustment may even exist for a wide applicable temperature range, for example from -<NUM> up to temperatures higher than <NUM>.

In the following, further improvements of the invention are described. The additional improvements may be combined independently of each other, depending on whether a particular advantage of a particular improvement is needed in a specific application.

According to a first advantageous improvement, the second material may be stabilized by a stabilizing material, the stabilizing material being an oxide of at least one of the following elements: yttrium, cerium, tantalum and niobium. However, also other elements may be used. It is also not excluded that a material, which is not an oxide, is used as a stabilizing material. The stabilized second material preferably has a basically tetragonal or cubic crystal structure. Thereby, a stable and homogeneous CTE may be achieved, even over a wide temperature range.

According to another advantageous improvement, the substrate may further comprise an insulating layer between the measuring structure and the remaining substrate. Due to the stabilizing material in the second material, the substrate may be electrically conductive, at least in parts. In particular, the material may have an ion conductivity that cannot be neglected. This ion conductivity may result from the difference in valences that creates voids or excess oxygen. However, an insulating layer between the measuring structure and the remaining substrate may overcome this problem. The insulating layer may preferably predominantly contain at least one of aluminum oxide, spinel (magnesium aluminate), and magnesium titanate. However, the insulating layer may also be made from other materials.

The measuring structure is preferably basically made from platinum, in particular made from pure platinum. A measuring structure made from platinum provides a reliable temperature-dependent resistance measurement. In the alternative, the measuring structure may for example be made from a platinum alloy, nickel, a nickel alloy, iridium or an iridium alloy.

According to another advantageous improvement, the substrate is a multilayered substrate made from a plurality of layers, at least one layer predominantly containing the first material and at least one layer predominantly containing the stabilized second material, wherein the at least one layer predominantly containing the first material and the at least one layer predominantly containing the stabilized second material are disposed one over another, and wherein the coefficient of thermal expansion of the plurality of layers deviates by less than <NUM> % from the coefficient of thermal expansion of the measuring structure.

Preferably, the CTE of the layer predominantly containing the first material is different from the CTE of the layer predominantly containing the stabilized second material, wherein the CTE of the plurality of layers is adjusted by the numbers and/or thicknesses of the layers containing the first and stabilized second material, respectively.

In order to achieve a homogeneous substrate, the layers containing the first material and the stabilized second material are preferably disposed over on another in an alternating manner. A total number of the layers may be between <NUM> and <NUM> layers.

In order to increase the accuracy of the matching between the CTEs of the substrate and the measuring structure made from platinum, a sum of thicknesses of the layers containing the stabilized second material is preferably approximately <NUM>% of a sum of the thicknesses of the layers containing the first material.

A thickness of each layer is preferably between <NUM> and <NUM>, more preferable between <NUM> and <NUM>.

The first material may contain aluminum oxide, spinel, yttrium aluminum garnet or a mixture of two or three of these materials. The term aluminum oxide refers to the material described by sum formula Al<NUM>O<NUM>. The term spinel refers to a material described by the sum formula MgAl<NUM>O<NUM>. The term yttrium aluminum garnet (known as "YAG") refers to the material Y<NUM>Al<NUM>[AlO<NUM>]<NUM>, described by sum formula Y<NUM>Al<NUM>O<NUM>.

The term zirconium dioxide, which is the main constituent of the stabilized second material, refers to ZrO<NUM>. The term hafnium dioxide refers to HfO<NUM>.

In the alternative to a multilayered substrate, the substrate may have a grain structure formed by a mixture of grains made from the first material and grains made from the stabilized second material.

The ratio of the grains with both materials may be adjusted in order to define the CTE of the substrate.

The substrate may preferably be produced by tape casting. Hence, a material is brought onto a carrier and a knife is moved along the material, bringing the material for the substrate into shape. If the substrate is made from a grain structure, a material containing a mixture with grains of both materials is used and shaped as a single layer. If a multilayered substrate is needed, the first material and the stabilized second material are tape casted one over the other in repeated cycles.

The resistance temperature sensor element according to the invention may be part of a resistance thermometer.

In the following, the invention and its improvements are described in greater detail using exemplary embodiments and with reference to the drawings. As described above, the various features shown in the embodiments may be used independently of each other in specific applications.

In the following figures, elements having the same function and/or the same structure will be referenced by the same reference signs.

A sensor assembly <NUM> according to a preferred embodiment is shown in <FIG>. The sensor assembly <NUM> can be used for a resistance temperature sensor element <NUM>, which is shown in <FIG>.

The sensor assembly <NUM> comprises a substrate <NUM> and a measuring structure <NUM> disposed on the substrate <NUM>.

The substrate <NUM> of the first embodiment is a multilayered substrate <NUM>, consisting of a plurality of layers.

The resistance temperature sensor element <NUM> may further comprise additional elements, of which only some are exemplarily shown in <FIG>. The resistance temperature sensor element <NUM> may, in addition to the assembly <NUM>, be provided with lead wires <NUM> that electrically connect the measuring structure <NUM>.

In addition, the resistance temperature sensor element <NUM> may comprise a cover layer <NUM>, which covers at least the measuring structure <NUM>. The cover layer <NUM> may also cover at least parts of the lead wires <NUM>. The cover layer <NUM> may be made from one or multiple glass materials.

In the following, the multilayered substrate <NUM> is described. The multilayered substrate <NUM> comprises at least one layer <NUM> that is predominantly made from a first material <NUM>. The first material <NUM> is at least one of aluminum oxide and yttrium aluminum garnet. The first material <NUM> may, hence, also be a mixture of both materials.

The multilayered substrate <NUM> also comprises at least one layer <NUM>, which is predominantly made from a stabilized second material <NUM>. The stabilized second material <NUM> may comprise at least one of zirconium dioxide and hafnium dioxide.

In order to form the stabilized second material <NUM>, at least one stabilizing material <NUM> is added to a raw second material <NUM>. The raw second material <NUM> is preferably at least one of zirconium dioxide and hafnium dioxide. Due to the addition of the stabilizing material <NUM> to the second material <NUM>, the stabilized second material <NUM> is formed.

The stabilized second material <NUM> may have a stabilized crystal structure, in particular in a desired temperature range. The stabilizing material <NUM> is indicated by circles in <FIG>. These circles shall only show that the stabilizing material <NUM> is dissolved in the raw second material <NUM> but does not indicate any structure.

The stabilizing material <NUM> is preferably an oxide of an element having a valence different from <NUM>, in particular <NUM> or <NUM>. For the raw second material <NUM>, the valence is four.

The layers <NUM> and <NUM> are disposed one over another, preferably in an alternating manner.

The stabilizing material <NUM> is preferably an oxide of yttrium, cerium or niobium.

The stabilizing material <NUM> preferably stabilizes a tetragonal and/or cubic crystal structure in the stabilized second material <NUM>. In particular, the stabilizing material <NUM> stabilizes a tetragonal and/or cubic crystal structure of the zirconium dioxide and/or the hafnium dioxide.

A total number of layers <NUM> and <NUM> is preferably between <NUM> and <NUM> layers. Of course, the multilayered substrate <NUM> may also contain less than <NUM> or more than <NUM> layers.

The layers <NUM> and <NUM> may have different thicknesses. Preferably, a thickness <NUM> of the layers <NUM> is smaller than a thickness <NUM> of the layers <NUM>.

Preferably, all layers <NUM> containing the first material <NUM> each have the same thickness <NUM>, and the layers <NUM> containing the stabilized second material <NUM> are each of the same thickness <NUM>, respectively. However, this is not mandatory.

Not all layers <NUM> containing the first material <NUM> must have the same thickness <NUM>. As well, the layers <NUM> containing the stabilized second material <NUM> must not all have the same thickness <NUM>.

In a preferred embodiment, the thicknesses <NUM> and <NUM> are between <NUM> and <NUM>.

The measuring structure <NUM> has a coefficient of thermal expansion (CTE) <NUM>. The multilayered substrate <NUM> has a CTE <NUM>. The coefficients of thermal expansion (CTEs) <NUM> and <NUM> are matched such that they differ about less than <NUM>% from each other.

The measuring structure <NUM> is preferably made from platinum. The CTE of platinum is <NUM> ppm/K. Hence, the CTE <NUM> of the substrate <NUM>, here the multilayered substrate <NUM>, preferably deviates not more than <NUM>% from the CTE of platinum and is preferably between <NUM> and <NUM> ppm/K.

More preferably, the CTE of the substrate <NUM> / multilayered substrate <NUM> deviates less than <NUM>% from the CTE of the measuring structure <NUM>.

The CTE <NUM> of the substrate <NUM> / multilayered substrate <NUM> is adjusted by the numbers and thicknesses <NUM> and <NUM> of the layers <NUM> and <NUM>. The materials <NUM> and <NUM> forming the layers <NUM> and <NUM> both have different CTEs.

Preferably, the CTE <NUM> of the first material <NUM> is about <NUM> ppm/K, whereas the CTE <NUM> of the stabilized second material <NUM> is about <NUM> ppm/K.

Depending on the numbers and thicknesses <NUM> and <NUM> of the layers <NUM> and <NUM>, an overall CTE about that of platinum, namely <NUM> ppm/K, can be achieved.

This good matching is achieved due to the fact that the stabilizing material <NUM> in the stabilized second material <NUM> stabilizes the crystal structure of this material such that the CTE of the stabilized second material <NUM> is around <NUM> ppm/K in all crystal directions and over the desired temperature range.

Just by way of example, the sum of thicknesses <NUM> of the layers <NUM> may be around <NUM>% of a sum of the thicknesses <NUM> of the layers <NUM>.

Since the stabilizing material <NUM> in the stabilized second material <NUM> may lead to a conductive multilayered substrate <NUM>, an additional insulating layer <NUM> may be present between the measuring structure <NUM> and the remaining layers <NUM> and <NUM> of the multilayered substrate <NUM>.

The insulating layer <NUM> electrically insulates the measuring structure <NUM> from electrically conductive layers of the remaining substrate <NUM> / multilayered Substrate <NUM>. The insulating layer <NUM> preferably contains aluminum oxide (Al<NUM>O<NUM>), spinel (MgAl<NUM>O<NUM>) or magnesium titanate (MgTiO<NUM>) or a mixture of two or three of these materials. The material for the insulating layer <NUM> is not limited to previous examples.

<FIG> shows another preferred embodiment of the invention. For the sake of brevity, only the difference to the first embodiment is described in detail.

Here, the substrate <NUM> is not a multilayered substrate <NUM>, but contains a grain structure <NUM>. The grain structure <NUM> contains grains <NUM> predominantly made from the first material <NUM> and grains <NUM> predominantly made from the stabilized second material <NUM>. However, it is not excluded that the substrate <NUM> contains additional materials.

In <FIG>, only some grains <NUM> and <NUM> are schematically indicated for explanatory reasons. The indicated grains are not representative for any size or shape or quantitative distribution.

The CTE <NUM> of the substrate <NUM> may be defined by adjusting the ratio of grains <NUM> and grains <NUM>.

Claim 1:
Sensor assembly (<NUM>) for a resistance temperature sensor element (<NUM>), the sensor assembly (<NUM>) comprising a substrate (<NUM>) and a measuring structure (<NUM>) disposed on the substrate (<NUM>), wherein the substrate (<NUM>) comprises:
at least one first material (<NUM>), the first material (<NUM>) being at least one of aluminum oxide, spinel (magnesium aluminate) and yttrium-aluminum-garnet,
at least one stabilized second material (<NUM>), the stabilized second material (<NUM>) being at least one of stabilized zirconium dioxide and stabilized hafnium dioxide, the stabilized second material (<NUM>) being stabilized by containing an oxide of an element having a valence different from four,
wherein a coefficient of thermal expansion (<NUM>) of the substrate (<NUM>) deviates by less than <NUM> % from a coefficient of thermal expansion (<NUM>) of the measuring structure (<NUM>)
characterized in that the stabilized second material is stabilized by containing an oxide of an element with valence three or five.