Patent Publication Number: US-9846138-B2

Title: Broadband lambda probe and production method for a broadband lambda probe

Description:
This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2014 200 481.0 filed on Jan. 14, 2014 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
     The disclosure relates to a broadband lambda probe. The disclosure further relates to a production method for a broadband lambda probe. 
     BACKGROUND 
     DE 101 63 942 A1 describes various embodiments of broadband lambda probes. Each of the broadband lambda probes has an oxygen pump cell comprising an outer pump electrode and an inner pump electrode and a Nernst concentration cell having a Nernst electrode and a reference electrode. Each of the broadband lambda probes also has at least one HC (hydrocarbon) electrode which is arranged on the outer face of said broadband lambda probe and by means of which a hydrocarbon content of an exhaust gas in an external environment of the respective broadband lambda probe is intended to be measurable. In order to protect against aggressive constituent parts of the exhaust gas, the at least one HC electrode is provided with a porous protective layer comprising zirconium dioxide. 
     SUMMARY 
     The disclosure provides a broadband lambda probe having the features of the disclosed subject matter, and a production method for a broadband lambda probe having the features of the disclosed subject matter. 
     The disclosure provides broadband lambda probes which are designed for combined detection of an oxygen partial pressure (or a lambda value) and of the at least one substance. The number of sensors required to analyze a substance composition within a volume can be reduced owing to the advantageous multifunctionality of the broadband lambda probes according to the disclosure. This also reduces the amount of work involved in arranging the sensors required to analyze the substance composition within the volume. The costs for further sensors which are not required can also be saved by means of the advantageous multifunctionality of the broadband lambda probes according to the disclosure. 
     A significant advantage of the broadband lambda probes according to the disclosure is also that the at least one capacitive sensor device which is integrated in the respective measurement hollow space is already protected against the aggressive environmental influences (for example gases, soot, ash and/or heat) which are present in the external environment of the respective broadband lambda probe owing to the advantageous arrangement of said sensor device. Therefore, the protective layer which is conventionally required according to the prior art under these conditions is dispensed with. Therefore, the present disclosure also contributes to reducing the costs of and the production outlay on multifunctional broadband lambda probes. 
     A sensitivity of the at least one capacitive sensor device is also improved owing to the integration into the measurement hollow space in the broadband lambda probe according to the disclosure. In the case of the broadband lambda probe according to the disclosure, the advantage of the precisely defined and relatively low oxygen concentration within the measurement hollow space can be utilized for the measurements executed by the at least one capacitive sensor device. Since there is virtually no oxygen in the respective measurement hollow space in general, there is no risk of oxygen-related cross-influences on the measurements which are executed by the at least one capacitive sensor device. The sensitivity of the at least one capacitive sensor device is therefore considerably increased. 
     In one advantageous embodiment, the at least one capacitive sensor device comprises in each case a first sensor electrode, in each case a second sensor electrode and in each case at least one dielectric which is present between the first sensor electrode and the second sensor electrode. Therefore, at least one MIM structure (metal-insulator-metal structure) can be integrated into the measurement hollow space in the broadband lambda probe as the at least one capacitive sensor device. Since an MIM structure can be formed in the measurement hollow space by means of method steps which can be executed in a simple manner, the integration of the at least one capacitive sensor device into the measurement hollow space in the broadband lambda probe can be executed without any significant additional outlay. In addition, at least one MIM structure can be easily formed in the measurement hollow space in the broadband lambda probe, without a large design of the broadband lambda probe being required for this purpose. 
     By way of example, the at least one dielectric of the respective capacitive sensor device at least comprises silicon dioxide, aluminum dioxide, hafnium oxide, tantalum oxide, zirconium oxide, silicon nitride, boron nitride, silicon carbide, tungsten silicide and/or tantalum silicide. Therefore, the at least one dielectric can be selected from a large number of cost-effective materials which are used relatively frequently (especially in semiconductor technology). 
     The at least one dielectric of the respective capacitive sensor device preferably comprises at least one material which has a bias-dependent permittivity and impedance at least at a temperature equal to an operating temperature of the broadband lambda probe. In particular, the at least one dielectric of the respective capacitive sensor device comprises at least barium titanate, lead zirconate titanate and/or barium strontium titanate as the at least one material. However, other materials which are polar at least at the operating temperature, in particular other ferroelectrics, can also be used instead of or in addition to the materials listed here. 
     In a further advantageous embodiment, the first sensor electrode of the respective capacitive sensor device, which first sensor electrode is oriented toward the measurement hollow space, comprises at least one catalytically active material. This may improve a sensitivity of the sensor element. 
     By way of example, the first sensor electrode of the respective capacitive sensor device, which first sensor electrode is oriented toward the measurement hollow space, comprises gold, platinum, aluminum, palladium, rhenium, ruthenium, iridium, titanium, titanium nitride, tantalum nitride and/or rhodium as the at least one catalytically active material. However, it should be noted that the catalytically active materials listed here are to be interpreted as being merely exemplary. 
     In a further advantageous embodiment, the second sensor electrode of the respective capacitive sensor device, which second sensor electrode is directed away from the measurement hollow space, comprises at least one semiconductor material. The first sensor electrode of the same capacitive sensor device may, if desired, also have at least one semiconductor material, especially the same semiconductor material as the second sensor electrode. 
     In particular, the second sensor electrode of the respective capacitive sensor device, which second sensor electrode is directed away from the measurement hollow space, can comprise silicon, germanium, gallium arsenide, indium phosphorus, silicon carbide and/or gallium nitride as the at least one semiconductor material. The semiconductor materials listed here can also be used for the first sensor electrode. Therefore, a large number of cost-effective semiconductor materials which are frequently used in semiconductor technology can be used for producing the at least one capacitive sensor device. 
     The capacitance/impedance of the respective capacitive sensor device can preferably be varied by means of changing a concentration of at least one hydrogen-containing gas and/or of at least one nitrogen oxide as the at least one substance, which concentration is present at the capacitive sensor device in the measurement hollow space. The at least one capacitive sensor device can therefore be used to detect/identify, for example, hydrogen, a large number of hydrocarbons, such as propene, ammonia, nitrogen monoxide and nitrogen dioxide in particular. In particular, the broadband lambda probe according to the disclosure can therefore be advantageously used to analyze an exhaust gas in an exhaust tract of a vehicle. 
     The at least one capacitive sensor device is advantageously electrically connected to an evaluation device of the broadband lambda probe by means of conductor tracks. In this case, the evaluation device is preferably designed to determine a leakage current, a capacitance, a bias-dependent impedance and/or a frequency-dependent impedance at the respective capacitive sensor device. As shown below by means of an example, the at least one capacitive sensor device can therefore be reliably used for a large number of detection and measurement methods. 
     The advantages cited above can also be implemented by executing the corresponding production method for a broadband lambda probe. The production method can be developed in line with the above-described embodiments of the broadband lambda probe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present disclosure will be explained below with reference to the figures, in which: 
         FIGS. 1 a  and 1 b    show schematic partial illustrations of an embodiment of the broadband lambda probe. 
         FIGS. 2 a  and 2 b    show coordinate systems illustrating a manner of operation of the above-described embodiment of the broadband lambda probe. 
         FIG. 3  shows a flowchart illustrating an embodiment of the production method for a broadband lambda probe. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 a  and 1 b    show schematic partial illustrations of an embodiment of the broadband lambda probe. 
     The broadband lambda probe partially illustrated in  FIG. 1 a    has an oxygen pump cell which comprises an outer pump electrode  10  and an inner pump electrode  12 . The oxygen pump cell having the pump electrodes  10  and  12  is designed to transfer oxygen from a measurement hollow space  14  in the broadband lambda probe to an external environment of the broadband lambda probe. However, since controlling the flow of oxygen across a porous diffusion barrier  16  by means of a voltage which is applied between the pump electrodes  10  and  12  is known from the prior art, the oxygen pump cell is not discussed in any detail here. 
     The measurement hollow space  14  is delimited from the external environment of the broadband lambda probe by the porous diffusion barrier  16 . To this end, an opening which is formed in a housing/substrate structure  18  of the broadband lambda probe and which extends from the external environment to the measurement hollow space  14  can be sealed off by means of the porous diffusion barrier  16  by way of example. 
     The broadband lambda probe also has a Nernst concentration cell having a Nernst electrode  20  and a reference electrode  22 . Since the interaction of the Nernst concentration cell with the oxygen pump cell is already known from the prior art, this is not discussed any further here. While the outer pump electrode  10  is arranged on an outer face of the broadband lambda probe, the inner pump electrode  12  and the Nernst electrode  20  are arranged directly at the measurement hollow space  14  and/or project at least partially into the measurement hollow space  14 . Therefore, there is direct contact between the at least one gas which is present in the measurement hollow space  14  and the electrodes  12  and  20 . The reference electrode  22  is arranged in/on a reference channel  24  which is schematically illustrated in  FIG. 1 a   . Since the ability to form the broadband lambda probe is not limited to a specific design/arrangement of the reference channel  24 , this is not discussed any further detail here. 
     In order to protect against aggressive environmental influences, the outer pump electrode  10  can be covered at least partially by a porous protective layer  26  and/or a non-porous protective layer  28 . Other outer surfaces of the broadband lambda probe can likewise be covered by one of the protective layers  26  and  28 . The reference electrode  22  can also be surrounded by at least one (porous or non-porous) protective layer  30 . 
     The broadband lambda probe comprises at least one capacitive sensor device  32  which is arranged in the broadband lambda probe in such a way that the at least one capacitive sensor device  32  directly adjoins the measurement hollow space  14  and/or projects at least partially into the measurement hollow space  14 . The at least one capacitive sensor device  32  is designed such that its capacitance can be varied by means of changing a concentration of at least one substance, which concentration is present at the respective capacitive sensor device  32  (in the measurement hollow space  14 ). The at least one capacitive sensor device  32  can therefore be used to identify/detect the at least one substance, in particular to measure a concentration of the at least one substance. Therefore, the broadband lambda probe can be used not only to measure the lambda value but also to identify/detect the at least one substance, in particular to determine the concentration of said substance. Other sensors which are conventionally additionally required to identify/detect the at least one substance can be saved owing to the advantageous multifunctionality of the broadband lambda probe. The need for dedicated installation spaces for gas sensors for detecting the at least one substance is also dispensed with owing to the multifunctionality of the broadband lambda probe. 
     Owing to the advantageous arrangement of said sensor device on/in the low-oxygen atmosphere present in the measurement hollow space  14 , there is no risk of oxygen-related cross-influences on the analyses/measurements which are executed by the at least one capacitive sensor device  32 . A sensitivity of the at least one capacitive sensor device  32  which is integrated in the measurement hollow space  14  is therefore particularly high. 
     In addition, owing to the advantageous integration of the at least one capacitive sensor device  32  into the measurement hollow space  14 , less aggressive environmental influences generally occur in the immediate environment of said capacitive sensor device. Therefore, there is no need to protect the at least one capacitive sensor device  32  which is integrated into the measurement hollow space  14  against an aggressive gas in the immediate environment of said capacitive sensor device. Therefore, protective layers which are conventionally required can be saved. In addition, the advantageous integration contributes to the increase in service life/useful time of the at least one capacitive sensor device  32 . 
     The capacitance of the respective/at least one capacitive sensor device  32  can preferably be varied by means of changing a concentration of at least one hydrogen-containing gas and/or of at least one nitrogen oxide as the at least one substance, said concentration being present at the capacitive sensor device  32  in the measurement hollow space  14 . Therefore, hydrogen (H 2 ), propene (C 3 H 6 ), ammonia (NH 3 ), nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ), for example, can be identified/detected as the at least one substance by means of the at least one capacitive sensor device  32 . In particular, the at least one capacitive sensor device  32  can be used to measure at least one concentration of the reducing or oxidizing gases listed here in this case. The broadband lambda probe can therefore be used especially to detect harmful gases which possibly occur in an exhaust tract of a motor vehicle. In particular, combustion of a fuel in an engine and/or exhaust gas after-treatment can therefore be efficiently regulated by means of the broadband lambda probe. 
       FIG. 1 b    shows an enlarged partial detail of  FIG. 1 a    with the at least one capacitive sensor device  32 . In the embodiment of  FIG. 1 b   , the illustrated capacitive sensor device  32  has a first sensor electrode  34 , a second sensor electrode  36  and at least one dielectric  38  which is present between the first sensor electrode  34  and the second sensor electrode  36 . The at least one capacitive sensor device  32  can therefore be designed as an MIM structure/MIM electrode (metal-insulator-metal structure, metal-insulator-metal electrode). A capacitive sensor device  32  of this kind can be integrated into the measurement hollow space  14  in a simple manner without the size of the respective broadband lambda probe being significantly increased for this purpose. 
     The first sensor electrode  34  is preferably oriented toward the measurement hollow space  14 . This can be understood to mean that a contact area  40  of the first sensor electrode  34 , which contact area is directed away from the second sensor electrode  36 , makes direct contact with the at least one gas which is present in the measurement hollow space  14 . In contrast, an arrangement/orientation which is directed away from the measurement hollow space  14  is preferred for the second sensor electrode  36 . Therefore, the second sensor electrode  36  is preferably fully shielded from the at least one gas present in the measurement hollow space  14  by means of the at least one dielectric  38  and the at least one sensor electrode  34 . By way of example, the second sensor electrode  36  can be arranged on/fastened to a substrate of the housing/substrate structure  18 . 
     The first sensor electrode  36  and the second sensor electrode  36  can be in the form of closed or in the form of porous metal layers. By way of example, the second sensor electrode  36  can be a closed metal layer, while the first sensor electrode  34  is of porous design. The first sensor electrode  34  which is oriented toward the measurement hollow space  14  can comprise at least one catalytically active material. In particular, the first sensor electrode  34  which is oriented toward the measurement hollow space  14  can comprise gold, platinum, aluminum, palladium, rhenium, ruthenium, iridium, titanium, titanium nitride, tantalum nitride and/or rhodium as the at least one catalytically active material. However, the materials listed here can also be used to form the second sensor electrode  36 . 
     By way of example, the second sensor electrode  36  which is directed away from the measurement hollow space  14  comprises at least one semiconductor material. In particular, silicon, germanium, gallium arsenide, indium phosphorus, silicon carbide and/or gallium nitride can be used as the at least one semiconductor material for forming at least the second sensor electrode  36 . However, if desired, the first sensor electrode  34  can also be formed from the semiconductor materials listed here or from another semiconductor material. 
     At least one of the sensor electrodes  34  and  36  can also be composed of a composite material (cermet) comprising at least one ceramic material in a metallic matrix. Therefore, a large number of different starting materials can be used to form wear-resistant sensor electrodes  34  and  36 . 
     The sensor electrodes  34  and  36  are preferably formed from different materials. However, the sensor electrodes  34  and  36  can also be formed from the same material or have the same material composition. Reference is made to DE 10 2009 029 621 A1 in respect of further ways of forming/arranging the sensor electrodes  34  and  36  of the at least one capacitive sensor device  32 . 
     The at least one dielectric  38  can comprise, for example, silicon dioxide, aluminum dioxide, hafnium oxide, tantalum oxide, zirconium oxide, silicon nitride, boron nitride, silicon carbide, tungsten silicide and/or tantalum silicide. However, the at least one dielectric  38  can also have a different electrically insulating material in addition or as an alternative to the materials listed here. The at least one dielectric  38  further preferably comprises at least one material which can be electrically polarized at a temperature equal to an operating temperature of the broadband lambda probe. In other words, the at least one material has a bias-dependent (or DC bias-dependent) permittivity and impedance at least at a temperature equal to the operating temperature of the broadband lambda probe. The at least one dielectric  38  can therefore be described as at least one thin dielectric layer which preferably contains a polarizable species. By way of example, the at least one dielectric  38  can comprise at least barium titanate, lead zirconate titanate and/or barium strontium titanate as the at least one material/the at least one polarizable species. 
     In addition, the broadband lambda probe can further have at least one integrated heating line  42  which is preferably embedded in an electrically insulating thermally conductive material  44 . On account of its integration into the measurement hollow space  14 , the at least one capacitive sensor device  32  can be heated together with other components of the broadband lambda probe by means of the at least one heating line  42  without additional outlay. Therefore, there is hardly any increase in energy consumption by the broadband lambda probe due to its being additionally equipped with the at least one capacitive sensor device  32 . 
     The at least one capacitive sensor device  32  can be electrically connected to an evaluation device of the broadband lambda probe by means of conductor tracks (not illustrated). In particular, laminated conductor tracks can be designed to establish contact between the at least one capacitive sensor device  32  and the broadband lambda probe. Therefore, conductor tracks of a type which is also used for the at least one further electrode  10 ,  12 ,  20  and  22  of the broadband lambda probe can be used to contact-connect the at least one capacitive sensor device  32 . Therefore, contact-connection of the at least one capacitive sensor device  32  is associated with hardly any additional outlay. 
     The evaluation device, not shown in  FIGS. 1 a  and 1 b   , can be designed to tap off at least one signal from the at least one capacitive sensor device  32  and/or to receive at least one signal which is provided by the at least one capacitive sensor device  32 , and to define at least one item of information relating to an ability to identify and/or relating to a concentration of the at least one substance taking into account the at least one signal. The evaluation device can then output the defined information and/or use said defined information to drive at least one apparatus, such as an injection apparatus for injecting a fuel into an internal combustion engine and/or an apparatus of an exhaust gas after-treatment system for example. 
     The evaluation device is preferably designed to determine a leakage current, a capacitance, a bias-dependent impedance and/or a frequency-dependent impedance at the at least one capacitive sensor device  32 . (A bias-dependent and frequency-dependent impedance can also be determined by means of the evaluation device.) The advantages of a design of the evaluation device of this kind are clear from the following example. 
       FIGS. 2 a  and 2 b    show coordinate systems for explaining a manner of operation of the above-described embodiment of the broadband lambda probe. 
     In the coordinate systems of  FIGS. 2 a  and 2 b   , the abscissas are in each case a time axis t (in minutes). A (standardized) capacitance C is indicated by means of the ordinate of the coordinate system of  FIG. 2 a   . An ordinate of the coordinate system of  FIG. 2 b    describes a concentration c of various gases (in percent). 
     In the example in  FIGS. 2 a  and 2 b   , the measurement hollow space is filled successively with nitrogen dioxide (NO 2 ) at a concentration of c(NO 2 ), ammonia (NH 3 ) at a concentration of c(NH 3 ), propene (C 3 H 6 ) at a concentration of c(C 3 H 6 ), nitrogen monoxide (NO) at a concentration of c(NO), and hydrogen (H 2 ) at a concentration of c(H 2 ). The three stages are then repeated by nitrogen dioxide. The measurement hollow space is flushed between each filling operation with various gases. 
     As shown by the capacitance C illustrated in  FIG. 2 a   , each of the listed gases, even at low concentrations, further causes a significant deviation in the capacitance C from its initial value. The broadband lambda probe is therefore suitable for identifying/detecting all of the listed gases. 
       FIG. 3  shows a flowchart for explaining an embodiment of the production method for a broadband lambda probe. 
     The production method can be executed, for example, to produce the above-described broadband lambda probe. However, the ability to execute the production method is not limited to the creation of this broadband lambda probe. 
     An oxygen pump cell of the broadband lambda probe with an outer pump electrode and an inner pump electrode is formed in a method step S 1 . This takes place in such a way that oxygen is transferred from a measurement hollow space in the broadband lambda probe to an external environment of the broadband lambda probe by means of the oxygen pump cell during operation of the broadband lambda probe. A Nernst concentration cell of the broadband lambda probe with a Nernst electrode and a reference electrode is formed in a method step S 2 . 
     In addition, at least one capacitive sensor device is formed in a method step S 3 , the capacitance of said sensor device being varied by means of changing a concentration of at least one substance, which concentration is present at the capacitive sensor device, during operation of the broadband lambda probe. The at least one capacitive sensor device is arranged in the broadband lambda probe in such a way that the at least one capacitive sensor device directly adjoins the measurement hollow space and/or projects at least partially into the measurement hollow space. 
     An advantageous way of forming the at least one capacitive sensor device as an MIM structure is already described above. The at least one dielectric of the MIM structure can be applied to at least one surface of the second sensor electrode by means of sputtering or atomic layer deposition for example. The at least one dielectric can also be deposited on the second sensor electrode by screen printing with subsequent sintering. The first sensor electrode can then be arranged on an outer face of the at least one dielectric, which outer face is directed away from the second sensor electrode. 
     In particular, the broadband lambda probe can also be produced using microsystem technology. The above-described production method can be executed to produce a single broadband lambda probe or to simultaneously produce a large number of broadband lambda probes. It should also be noted that the ability to execute the production method is not limited to compliance with a specific time sequence of method steps S 1  to S 3 . The method steps S 1  to S 3  can be carried out in any desired order or at the same time.