Abstract:
A temperature sensor includes a substrate, a platinum resistor arranged on at least one surface of the substrate, a protective layer covering at least a portion of the platinum resistor and a cover layer covering at least a portion of the protective layer, the cover layer including Al2O3, SiO2 and Y2O3. The cover layer may also include B2O3. A conductive wire may be electrically connected to the platinum resistor. A glass ceramic may be covering at least a portion of the conductive wire, platinum resistor, protective layer and cover layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 2013 112 493.3, filed Nov. 13, 2013, the entire contents of which are hereby incorporated by reference. 
     DESCRIPTION 
     Field of the Invention 
     The present invention generally relates to an improved temperature sensor. 
     BACKGROUND OF THE INVENTION 
     An embodiment of the invention is based on a temperature sensor having the features listed in the claims. Prior art temperature sensors are known from US 6,617,956 B1 and US 7,233,226 B2. 
     US 6,617,956 B1 and US 7,233,226 B2 disclose temperature sensors comprising a platinum resistor that is arranged on a substrate and covered by a protective ceramic layer of Al2O3. The thin protective layer is covered by a thicker covering layer that is a mixture of Al2O3, MgO and SiO2. The known temperature sensors can only be used at temperatures up to 1000° C. and show significant resistance drift after extended use at such elevated temperatures. 
     An object of the present invention is to show how the temperature resistance of such sensors can be improved and resistance drift lowered. This problem, and other problems, may be solved by a temperature sensor according to the claims. Further advantageous refinements of the invention may be the matter of the dependent claims. 
     SUMMARY OF THE INVENTION 
     An increase in temperature resistance is achieved with a temperature sensor comprising a substrate, a platinum resistor arranged on the substrate, a protective layer covering the platinum resistor, and a cover layer covering the protective layer in that the cover layer contains Al2O3, SiO2, and Y2O3. By adding Y2O3 to Al2O3 and SiO2 a cover layer can be provided that results in significantly reduced layer diffusion. The cover layer is thus less prone to pore formation and can provide a reliable seal up to higher temperatures. Moreover, reduced layer diffusion means that contact between material of the cover layer and the platinum resistor can be prevented up to higher temperatures and thus resistance shift reduced. 
     A rather small amount of Y2O3 is sufficient to improve the heat resistance of a layer that consists predominantly of Al2O3 and SiO2. For example, the cover layer may contain 5% by weight of Y2O3 or more. In an embodiment of the invention, the cover layer may contain 10% by weight of Y2O3 or more. Increasing the concentration of Y2O3 beyond 30% by weight does not improve the cover layer significantly and may not be economical. An embodiment of the invention may be that the cover layer contains less than 20% by weight of Y2O3. 
     Another embodiment of the invention may be that SiO2 and Al2O3 together add up to at least 50% by weight of the cover layer, for example 60% by weight or more. 
     Another embodiment of the invention may be that the cover layer contains more SiO2 by weight than Al2O3. For example, the cover layer can contain twice as much SiO2 by weight than Al2O3 or more. 
     Another embodiment of the invention is that the cover layer contains less Y2O3 by weight than Al2O3. Another embodiment of the invention is that the cover layer contains more SiO2 by weight than Y2O3. 
     The cover layer may contain at least 30% by weight of SiO2, for example 40% by weight or more. A SiO2 content of more than 70% is usually not advantageous. 
     The cover layer may contain at least 15% by weight of Al2O3, for example 20% weight or more. An Al2O3 content of more than 30% is usually not advantageous. 
     The cover layer may also contain B2O3, e.g. up to 25% by weight. For example, in an embodiment of invention the cover layer may contain 1% by weight to 20% by weight of B2O3. The cover layer may also contain additional additives, especially other oxides besides SiO2, Al2O3, Y2O3 and B2O3. In a possible embodiment of the invention, the content of any additional additives may be less than 20% by weight in total, for example not more than 10% by weight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate the invention. In such drawings: 
         FIG. 1  shows a schematical cross-section of an embodiment of a temperature sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a temperature sensor shown in  FIG. 1  comprises a substrate  1 , for example an alumina substrate. A platinum resistor  2 , which may be connected to a wire  3 , is arranged on the substrate  1 . The platinum resistor  2  is a resistive layer and may be made of any platinum metal or platinum metal based alloy. The platinum resistor  2  is covered by a protective layer  4 , for example a ceramic layer. The protective layer  4  can be made of alumina or other ceramic material. A cover layer  5  is arranged on top of the protective layer  4 . The cover layer  5  can be covered by an additional layer  6 , e.g. a glaze layer. A connection area of the wire  3  and the platinum resistor  2  may be covered by a glass ceramic  7  in order to secure and protect the connection between lead wire  3  and platinum resistor  2 . 
     The cover layer  5  may be a glass ceramic or glaze layer. The cover layer  5  contains SiO2, Al2O3, and Y2O3. For example, the cover layer  5  may contain 40 to 60% by weight of SiO2, 20 to 25% by weight of Al2O3, and 10 to 19% by weight of Y2O3. The cover layer  5  may also contain up to 20% by weight of B2O3, e.g. 5% to 20% by weight of B2O3, and up to 20% by weight of other components, especially other oxides. Such a temperature sensor can be used for measuring temperatures up to 1200° C. 
     The protective layer  4  can be applied by a vapour deposition method or as a green foil that is later fired. The cover layer  5  can be produced by a screen-printing method, for example. In the embodiment of  FIG. 1 , which is not to scale, the cover layer  5  is thicker than the protective layer  4 . If an additional layer  6  is placed on top of the cover layer  5 , this additional layer  6  may be even thicker than the cover layer  5 . Any additional layer  6  may be applied as a paste, e.g. by printing and later fired. The thickness of the various layers may not be critical for the functioning of the temperature sensor and may be chosen for manufacturing considerations. 
     REFERENCE NUMERALS 
     1 Substrate 
     2 Platinum resistor 
     3 Wire 
     4 Protective layer 
     5 Cover layer 
     6 Additional layer 
     7 Glass ceramic 
     Although several embodiments have been described in detail for purposes of illustration, various modifications may be made to each without departing from the scope and spirit of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.