Abstract:
A sensor element having a layered construction and configured to detect a physical property of a gas or a liquid includes a functional component situated in the interior, which functional component is connected electrically to a conductor element, which conductor element extends up to the outer surface or up into the surroundings of the sensor element. The sensor element has at least one sealing element which adjoins the functional component and/or the conductor element. The conductor element and the at least one sealing element are configured to be gas-tight at least regionally in the interior of the sensor element and are situated in such a way that the functional component is separated gas-tight from the surroundings of the sensor element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is directed to a sensor element containing a sealing element for a functional component. 
         [0003]    2. Description of Related Art 
         [0004]    A sensor element having a layered construction is known, for example, from published German patent application document DE 10 2004 025 949 A1, and has a printed conductor, which extends from an outer side of the sensor element through a feedthrough up into the interior of the sensor element. A cover layer is situated in the area of the feedthrough in such a way that a gas located outside the sensor element may only reach the interior of the sensor element via a diffusion path, which at least regionally runs parallel to the outer surface of the sensor element. A sensor element constructed in this way has the advantage that contaminants do not accumulate or only accumulate substantially less in the interior of the sensor element. 
         [0005]    Such a sensor element has the disadvantage that access of a gas which is located outside the sensor element, in particular an oxygenated exhaust gas or ambient air, to the functional component, which is situated in the interior of the sensor element, is possible, which results there in oxidative processes in particular at high temperatures, which may impair the function of the sensor element and contribute to premature aging and to the failure of the sensor element. These problems occur more extensively the baser and thus more reactive the material of the functional component. On the other hand, the use of such materials is attractive for cost reasons. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The sensor element according to the present invention has the advantage over the related art in that an access of a gas, which is located outside the sensor element, to the functional component is not possible, and therefore oxidation of the functional component may be prevented even at high temperatures and while employing base materials. For this purpose, the conductor element and the sealing element in the interior of the sensor element are at least regionally designed as gas-tight and are situated in such a way that the functional component is separated gas-tight from the surroundings of the sensor element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows a cross-sectional view of an end section of an example embodiment of a sensor according to the present invention. 
           [0008]      FIG. 2  shows a cross-sectional view of an end section of another example embodiment of a sensor according to the present invention. 
           [0009]      FIGS. 3 and 3   a  show a cross-sectional view of an end section of another example embodiment of a sensor according to the present invention. 
           [0010]      FIG. 4  shows a cross-sectional view of an end section of another example embodiment of a sensor according to the present invention. 
           [0011]      FIG. 5  shows a cross-sectional view of an end section of another example embodiment of a sensor according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    As a first exemplary embodiment of the present invention,  FIG. 1  shows a connection-side end section of a sensor element  20 , which is situated in a housing of a gas sensor (not shown) and is used, for example, for determining the oxygen concentration in an exhaust gas of an internal combustion engine (not shown) or the temperature of the exhaust gas. 
         [0013]    Sensor element  20  is constructed from ceramic layers  21 ,  22 ,  28 ,  29 , of which two are formed as a first and a second solid electrolyte film  21 ,  22  and contain yttrium-oxide-stabilized zirconium oxide (YSZ) and two are formed as an outer and an inner printed, electrically insulating layer  28 ,  29  and contain aluminum oxide. 
         [0014]    First and second solid electrolyte films  21 ,  22  are located above and below inner printed electrically insulating layer  29 . Outer printed electrically insulating layer  28  is situated above second solid electrolyte film  22 . Of course, the sensor element may have further layers for implementing functionalities of sensor element  20  which are known per se. These further layers may be made of ceramic material, for example. 
         [0015]    A functional component  31 , which is composed of an electrical resistance heater and a supply line  131  to the electrical resistance heater, is located inside inner printed electrically insulating layer  29 . The electrical resistance heater causes, together with external wiring (not shown), the heating of sensor element  20  to temperatures up to significantly greater than 650° C. Supply line  131  to the electrical resistance heater extends up to the connection-side end section of sensor element  20 , while the electrical resistance heater is situated in the diametrically opposing, measurement-side end section of sensor element  20  (not shown in  FIG. 1 ). The material of which functional component  31  is made in this example has a high palladium proportion, for example, a proportion of greater than 50 percent by weight. Alternatively, other materials which also oxidize at temperatures greater than 650° C. in the presence of oxygen come into consideration. 
         [0016]    Since the material, of which functional component  31  is made in this example, oxidizes at the operating temperatures of functional component  31  of greater than 650° C. in the presence of oxygen, it is provided that functional component  31  is separated gas-tight from surroundings  500  of sensor element  20 . 
         [0017]    In the area of the connection-side end section, sensor element  20  has a feedthrough  51  which extends, starting from functional component  31 , through parts of inner printed electrically insulating layer  29 , through second solid electrolyte film  22 , and through outer printed electrically insulating layer  28  up to outer surface  100  of sensor element  20 . The feedthrough has the form of a circular cylinder having a diameter of 0.3 mm to 1.5 mm, preferably 0.5 mm to 1.2 mm, whose axis is perpendicular to layers  21 ,  22 ,  28 ,  29 . Of course, it would also be possible to give the footprint of feedthrough  51  an oval or polygonal shape and/or to situate feedthrough  51  at a different angle to layers  21 ,  22 ,  28 ,  29 . A gas-tight, electrical insulation  61 , which contains aluminum oxide, aluminum-magnesium spinel, or forsterite, for example, and has a thickness of 2 μm-100 μm, preferably 5 μm-50 μm, is applied to the wall of feedthrough  51 , which is in the form of a cylindrical jacket. Insulation  61  thus has the form of a hollow cylinder. 
         [0018]    A conductor element  41  is connected electrically conductive to functional component  31  in feedthrough  51  and in the interior of the hollow cylinder which is formed by insulation  61  with the aid of a conductive fixing attachment  75 , an electrically conductive and mechanically fixing compound. Conductor element  41  is a platinum wire  141 , which extends inside feedthrough  51  along its axis and up into surroundings  500  of sensor element  20 . Platinum wire  141  has a diameter of 50 μm-250 μm and has a gas-tight design. The remaining space of feedthrough  51 , i.e., the space in the interior of the hollow cylinder formed by insulation  61 , which is not occupied by conductive fixing attachment  75  or by platinum wire  141 , is filled up by a sealing element  71 , which is made of a gas-tight and electrically insulating glass or glass-ceramic compound  171 . Glass or glass-ceramic compound  171  is SiO 2 -based or phosphate-based having further proportions of Al 2 O 3 , MgO, CaO, and/or B 2 O 3 . Furthermore, glass or glass-ceramic compound  171  may contain proportions of ZnO, SrO, BaO, La 2 O 3 , TiO 2 , and Na 2 O totaling less than 50 percent by weight, preferably less than 10 percent by weight. The coefficient of thermal expansion of employed glass or glass-ceramic compound  171  is between 4.5*10 −6 /K and 12*10 −6 /K for reasons of adaptation to the thermal expansion of the surrounding ceramics. 
         [0019]    Thick-layer processes which are known per se are used for producing sensor element  20  according to the first exemplary embodiment, for example, screen printing, transfer printing, and through suction processes. Glass or glass-ceramic compound  171  is dispensed, prepared in powdered form, which is unpressed or pressed into shape, or in paste form, into feedthrough  51  and fired using a subsequent heating process, glass or glass-ceramic compound  171  becoming gas-tight through fusion and subsequent solidification. The firing temperature of the employed glasses is in the range from 900° C.-1400° C. The fired glass or the fired glass ceramic is high-temperature resistant and has a glass transition point of greater than 750° C. 
         [0020]      FIG. 2  shows a second exemplary embodiment of the present invention, which differs from the first exemplary embodiment in that conductor element  41  is designed as a metal core  241 . Metal core  241  has the form of a circular cylinder having a diameter of 80 μm-400 μm and terminates flatly with outer surface  100  of sensor element  20 , like sealing element  71 . Metal core  241  is connected mechanically and electrically conductive with the aid of a conductive fixing attachment  75  to functional component  31 , as in the first exemplary embodiment of platinum wire  141 , or a ductile metal layer  275  made of gold or nickel, which is 10 μm-200 μm thick, is located between metal core  241  and functional component  31 . Metal core  241  contains platinum or nickel or is made of a chromium-nickel steel and has a gas-tight design. 
         [0021]    A planar contact element  42 , which is connected mechanically and electrically conductive to metal core  241 , and is used for contacting the sensor element with an analysis and/or supply unit (not shown), is situated on outer surface  100  of sensor element  20 . 
         [0022]      FIGS. 3 and 3   a  show a third exemplary embodiment of the present invention, which differs from the second exemplary embodiment in that contact element  42  is not only situated on outer surface  100  of sensor element  20 , but rather also protruding into feedthrough  51  and into the interior of the hollow cylinder formed by insulation  61 . Insulation  61  is composed and positioned as in the preceding examples, and is gas-tight in particular and takes over the function of sealing element  71  in this exemplary embodiment. Conductor element  41  includes a gas-tight, conductive filling  341  in this exemplary embodiment, which is connected electrically conductive to contact element  42  and to functional component  31 . Gas-tight, conductive filling  341  fills up the entire cross-section of feedthrough  51 , together with insulation  61 , below the protrusion of contact element  42 . Gas-tight, conductive filling  341  is made of a material which contains 5-90 percent by volume, preferably 10-50 percent by volume, platinum or platinum group metals (nickel, palladium, platinum) and also a glass or glass ceramic phase, whose composition corresponds to the composition of glass or glass ceramic compound  171  of the first exemplary embodiment, for example, and optionally contains alloy elements having a low sintering point, such as gold or silver. Filling  341  has a closed porosity. 
         [0023]    In  FIG. 3   a , dimension a is the diameter of the hollow cylinder formed by insulation  61 , dimension b is the length of the protrusion of contact element  42  into the interior of the hollow cylinder formed by insulation  61 , dimension c is the width of this protrusion, dimension d is the thickness of layer  28 , dimension e is the thickness of layer  29 , and dimension f is the length of the protrusion of contact element  42  into gas-tight, conductive filling  341 . Dimensions a, b, c, d, e, and f are selected as follows: a=900 μm, b=190 μm, c=30 μm, d=25 μm, e=45 μm, and f=135 μm. Alternatively, the dimensions are selected within the following limits: 300 μm&lt;a&lt;1500 μm, 20 μm&lt;b&lt;300 μm, 2 μm&lt;c&lt;100 μm, 5 μm&lt;d&lt;50 μm, 5 μm&lt;e&lt;70 μm, 20 μm&lt;f&lt;300 μm, preferably: 500 μm&lt;a&lt;1200 μm, 100 μm&lt;b&lt;200 μm, 5 μm&lt;c&lt;50 μm, 20 μm&lt;d&lt;30 μm, 40 μm&lt;e&lt;50 μm, 50 μm&lt;f&lt;200 μm. 
         [0024]      FIG. 4  shows a fourth exemplary embodiment of the present invention, which differs from the third exemplary embodiment in that the protrusion of contact element  42  extends up to the base of feedthrough  51  and is connected electrically conductive there to functional component  31 . The protrusion of contact element  42  has a gas-tight design and takes over the function of conductor element  41  in this exemplary embodiment and is designed as a hollow cylinder. The interior of the protrusion of contact element  42  is filled up by sealing element  71 , which is made up of a glass or glass-ceramic compound  471  in this example. The composition of glass or glass-ceramic compound  471  corresponds to the composition of glass or glass-ceramic compound  171  from the first exemplary embodiment. The part of contact element  42  which is situated on outer surface  100  has a thickness of 2 μm-100 μm, preferably 5 μm-25 μm in this exemplary embodiment. 
         [0025]    In an alternative specific embodiment of exemplary embodiments 1 through 4, layer  22  is made up of a gas-tight, electrically insulating material, which predominantly contains aluminum oxide and has a closed porosity, for example. Layer  22  takes over the function of sealing element  71  in this variant and conductor element  41  may be led through layer  22  in direct contact therewith. Insulation  61  may be dispensed with in this case. 
         [0026]    The gas-tight separation of functional component  31  from surroundings  500  of sensor element  20  may also take place in that conductor element  41  runs at least partially within the same layer plane as functional component  31  and is gas-tight there, and sealing element  71  is situated at least partially within the same layer plane as a layer  29 , which encloses functional component  31 , and is gas-tight there. It is advantageously possible in this case to achieve a gas-tight separation of functional component  31  from surroundings  500  of sensor element  20  without a feedthrough  51  through sensor element  20  which is used for contacting functional component  31 , having to be gas-tight. The fifth exemplary embodiment of the present invention, which is based on this idea, is shown in  FIG. 5 . 
         [0027]      FIG. 5  shows a connection-side end section of a sensor element  20 , which is situated in a housing of a gas sensor (not shown) and is used, for example, for determining the oxygen concentration in an exhaust gas of an internal combustion engine (not shown) or the temperature of the exhaust gas. 
         [0028]    Sensor element  20  is constructed from ceramic layers  21 ,  22 ,  28 ,  29 , of which two are designed as a first and a second solid electrolyte film  21 ,  22  and contain yttrium-oxide-stabilized zirconium oxide (YSZ) and two are designed as an outer and an inner printed, electrically insulating layer  28 ,  29  and contain aluminum oxide. 
         [0029]    First and second solid electrolyte films  21 ,  22  are located above and below inner printed electrically insulating layer  29 . Outer printed electrically insulating layer  28  is situated above second solid electrolyte film  22 . 
         [0030]    A functional component  31 , which is composed of an electrical resistance heater and a supply line  131  to the electrical resistance heater, is located inside inner printed electrically insulating layer  29 . The electrical resistance heater causes, together with an external wiring (not shown), the heating of sensor element  20  to temperatures greater than 650° C. Supply line  131  to the electrical resistance heater extends at least close to the connection-side end section of sensor element  20 , while the electrical resistance heater is situated in the diametrically opposing, measurement-side end section of sensor element  20  (not shown in  FIG. 5 ). The material of which functional component  31  is made of in this example has a high palladium proportion, for example, a proportion of greater than 50 percent by weight. 
         [0031]    In the area of its connection-side end section, sensor element  20  has a feedthrough  51 , which extends from the layer plane in which functional component  31  is located up to outer surface  100  of sensor element  20 . A gas-tight, electrical insulation  61  is applied to the wall of feedthrough  51 , in the form of a cylindrical sheath. 
         [0032]    A contact element  42  extends from outer surface  100  of sensor element  20  along the inner side of insulation  61  up to the layer plane in which functional component  31  lies. Contact element  42  is designed as a hollow cylinder inside feedthrough  51  with an interior remaining free. Contact element  42  is connected electrically conductive to functional component  31 . 
         [0033]    Since the material of which functional component  31  is made of oxidizes in the presence of oxygen at operating temperatures of functional component  31  of greater than 650° C., functional component  31  is separated gas-tight from surroundings  500  of sensor element  20 . 
         [0034]    For this purpose, a part  29   a  of inner electrically insulating printed layer  29  is gas-tight in an area situated laterally around feedthrough  51 . This part  29   a  of inner electrically insulating printed layer  29  takes over the function of sealing element  71 . Furthermore, a gas-tight supply line  31   a , preferably having high platinum content, which electrically connects contact element  42  and functional component  31  to one another, is located in the layer plane of functional component  31 . This gas-tight supply line  31   a  takes over the function of conductor element  41 . The gas-tightness of sealing element  71  and conductor element  41  is achieved by a closed porosity or by sintering additives of a glass or glass-ceramic phase or by adding alloy elements which sinter at low temperatures, such as gold or silver. 
         [0035]    The area which is situated laterally around feedthrough  51  extends, starting from the outer edge of feedthrough  51 , to a width of between 300 μm and 5000 μm. In one specific embodiment it may also be provided that functional component  31  and gas-tight supply line  31   a  overlap on a length of up to 1 mm. 
         [0036]    All exemplary embodiments allow the use of a material which oxidizes under the influence of oxygen for functional component  31  at operating temperatures of up to greater than 650° C. This material may be a relatively cost-effective noble metal in comparison to platinum, such as palladium or gold. Furthermore, the use of metals which are not noble metals is possible, such as nickel, tungsten, molybdenum, titanium, tantalum, niobium, iron, or chromium. For this purpose, it is to be noted that if these materials are used, oxidations of the materials used for the functional components are also to be prevented during the manufacturing process. For this purpose, in particular during the sintering procedure, the use of a reducing atmosphere is advantageous, in particular the gases argon and nitrogen having a volume proportion of up to 5% hydrogen. 
         [0037]    If materials are used for functional component  31  which react with aluminum oxide, for example, various metals (atomic type Me) do this, in that they react to form MeAl 2 O 4  (spinel) at high temperatures, direct contact between functional component  31  and aluminum oxide is to be prevented, for example, by providing a diffusion barrier layer. In the case of nickel, it may be made of zirconium oxide, for example. Since the coefficient of thermal expansion of the relevant materials sometimes significantly deviates from the coefficient of thermal expansion of the employed ceramics, it may be advantageous for the material of which functional component  31  is made of to have a ceramic second phase (cermet), whereby it is possible to bring the coefficients of thermal expansion into harmony. 
         [0038]    A further possibility is the use of carbon in the form of carbon nanotubes as the material for functional component  31 . The material containing the carbon nanotubes is advantageously processed as a paste for this purpose, for example, with the aid of screenprinting. A debinding of this paste may be performed in an oxygenated atmosphere, the sintering process is to be performed in a protective gas atmosphere. Since material containing carbon nanotubes having a high specific conductance is presently only available to a limited extent, it may be advantageous to use materials in functional components  31  which have carbon in the form of carbon nanotubes (for example, as a heating resistor of an electrical heater) in addition to other materials (for example, having platinum in the supply lines of this heater). If the heating resistor of an electrical heater is constructed by carbon in the form of carbon nanotubes, the possibility exists of designing this heating resistor as planar, for example, in an area whose edge lengths are greater than 2 mm.