US20020023838A1 - Gas sensor and corresponding production method - Google Patents
Gas sensor and corresponding production method Download PDFInfo
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- US20020023838A1 US20020023838A1 US09/509,270 US50927000A US2002023838A1 US 20020023838 A1 US20020023838 A1 US 20020023838A1 US 50927000 A US50927000 A US 50927000A US 2002023838 A1 US2002023838 A1 US 2002023838A1
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- measuring electrode
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- 238000004519 manufacturing process Methods 0.000 title claims abstract description 6
- 239000011253 protective coating Substances 0.000 claims abstract description 22
- 238000000034 method Methods 0.000 claims abstract description 13
- 230000008021 deposition Effects 0.000 claims abstract description 11
- 239000011148 porous material Substances 0.000 claims abstract description 10
- 239000007784 solid electrolyte Substances 0.000 claims abstract description 9
- 238000005246 galvanizing Methods 0.000 claims abstract description 7
- 238000004017 vitrification Methods 0.000 claims abstract description 7
- 239000000463 material Substances 0.000 claims description 15
- 229910052751 metal Inorganic materials 0.000 claims description 11
- 239000002184 metal Substances 0.000 claims description 11
- 239000011195 cermet Substances 0.000 claims description 10
- 239000000919 ceramic Substances 0.000 claims description 7
- 238000010438 heat treatment Methods 0.000 claims description 7
- 239000000203 mixture Substances 0.000 claims description 7
- 150000002739 metals Chemical class 0.000 claims description 4
- 229910045601 alloy Inorganic materials 0.000 claims description 2
- 239000000956 alloy Substances 0.000 claims description 2
- 239000010953 base metal Substances 0.000 claims description 2
- 239000010970 precious metal Substances 0.000 claims description 2
- 239000010410 layer Substances 0.000 description 72
- 239000007789 gas Substances 0.000 description 28
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 16
- 239000011888 foil Substances 0.000 description 12
- 239000010931 gold Substances 0.000 description 12
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 11
- 229910052737 gold Inorganic materials 0.000 description 11
- 239000001301 oxygen Substances 0.000 description 11
- 229910052760 oxygen Inorganic materials 0.000 description 11
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 10
- 239000010948 rhodium Substances 0.000 description 7
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 6
- 229910052703 rhodium Inorganic materials 0.000 description 6
- MHOVAHRLVXNVSD-UHFFFAOYSA-N rhodium atom Chemical compound [Rh] MHOVAHRLVXNVSD-UHFFFAOYSA-N 0.000 description 6
- 229910052697 platinum Inorganic materials 0.000 description 5
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 description 4
- MCMNRKCIXSYSNV-UHFFFAOYSA-N Zirconium dioxide Chemical compound O=[Zr]=O MCMNRKCIXSYSNV-UHFFFAOYSA-N 0.000 description 4
- 230000003197 catalytic effect Effects 0.000 description 4
- 229930195733 hydrocarbon Natural products 0.000 description 4
- 150000002430 hydrocarbons Chemical class 0.000 description 4
- 229910052759 nickel Inorganic materials 0.000 description 3
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 2
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 2
- 229910004042 HAuCl4 Inorganic materials 0.000 description 2
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 2
- 238000000137 annealing Methods 0.000 description 2
- 239000010949 copper Substances 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 239000003792 electrolyte Substances 0.000 description 2
- 229910052741 iridium Inorganic materials 0.000 description 2
- GKOZUEZYRPOHIO-UHFFFAOYSA-N iridium atom Chemical compound [Ir] GKOZUEZYRPOHIO-UHFFFAOYSA-N 0.000 description 2
- 239000002346 layers by function Substances 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 229910052763 palladium Inorganic materials 0.000 description 2
- 229910052709 silver Inorganic materials 0.000 description 2
- 239000004332 silver Substances 0.000 description 2
- 229910021604 Rhodium(III) chloride Inorganic materials 0.000 description 1
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- 239000010405 anode material Substances 0.000 description 1
- 229910052797 bismuth Inorganic materials 0.000 description 1
- JCXGWMGPZLAOME-UHFFFAOYSA-N bismuth atom Chemical compound [Bi] JCXGWMGPZLAOME-UHFFFAOYSA-N 0.000 description 1
- 229910052593 corundum Inorganic materials 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 239000007772 electrode material Substances 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 150000002343 gold Chemical class 0.000 description 1
- 150000003058 platinum compounds Chemical class 0.000 description 1
- SONJTKJMTWTJCT-UHFFFAOYSA-K rhodium(iii) chloride Chemical compound [Cl-].[Cl-].[Cl-].[Rh+3] SONJTKJMTWTJCT-UHFFFAOYSA-K 0.000 description 1
- 229910052707 ruthenium Inorganic materials 0.000 description 1
- 150000003839 salts Chemical class 0.000 description 1
- 239000000523 sample Substances 0.000 description 1
- 238000007650 screen-printing Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 229910002058 ternary alloy Inorganic materials 0.000 description 1
- 239000010936 titanium Substances 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 229910001845 yogo sapphire Inorganic materials 0.000 description 1
Images
Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4075—Composition or fabrication of the electrodes and coatings thereon, e.g. catalysts
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4071—Cells and probes with solid electrolytes for investigating or analysing gases using sensor elements of laminated structure
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4077—Means for protecting the electrolyte or the electrodes
Definitions
- the invention relates to a gas sensor according to the preamble of claim 1 and a method for manufacturing the gas sensor.
- a gas sensor having a measuring electrode and a reference electrode arranged on a solid electrolyte is known from European Patent 466 020 A.
- the measuring electrode is made of a platinum compound or a ternary alloy that includes platinum, gold, nickel, copper, rhodium, ruthenium, palladium or titanium.
- the materials may be applied to the solid electrolyte as multiple layers, the alloying step being carried out after the materials are applied.
- the gas sensor according to the present invention having the characterizing features set forth in claim 1 has the following advantage: a vitrified sensor element basic body can be used, the further layer being integrated via just one additional deposition step following vitrification.
- the outer electrode of the sensor element basic body can be modified following vitrification.
- the sensor element of a Nernst-type lambda sensor for example, can be used as the sensor element basic body, it being possible to transform the outer electrode into a mixed potential electrode by making certain modifications.
- materials that would not withstand the high temperature at which vitrification is carried out can be used as the further layers.
- a further advantage is that the further layer system, which is directly adjacent to the electrically conductive base layer, does not completely fill the pores of the porous protective coating.
- the porous protective coating continues to provide effective protection, and sufficient gas can access the three-phase boundary.
- the material used as the further layer may be used to modify the functional characteristics of the electrode of the gas sensor in a specific manner.
- this modification may define the specific gas selectivity of the sensor and/or its position within the control system.
- a particularly advantageous sensor designed for mixed potentials can be achieved if the layer system is subjected to a thermal additional treatment following deposition of the further layer.
- a thermal additional treatment following deposition of the further layer For example, in the case of a Pt/Au electrode a temperature range of 1200° C. ⁇ 100° C. is favorable. At this temperature, the metal atoms of the further layer diffuse into the metal of the adjacent base layer.
- a further advantage is that a cermet layer is used as the electrically conductive base layer which, thanks to its ceramic component, creates a solid join with the solid electrolyte when the ceramic body is vitrified. Furthermore, by creating a plurality of further layers and choosing the layer material appropriately one can specify the selectivity and also modify the catalytic activity of the electrode with even greater precision.
- FIG. 1 shows a section through a gas sensor according to the present invention
- FIG. 2 shows an enlarged sectional view of a first exemplary embodiment of an electrode of the gas sensor according to the present invention.
- FIG. 3 shows an enlarged sectional view of a second exemplary embodiment of an electrode of the gas sensor according to the present invention.
- FIG. 1 shows a gas sensor having a sensor element basic body 10 whose structure corresponds to that of a Nernst-type oxygen sensor (lambda sensor).
- Basic body 10 includes, for example, a plurality of ceramic solid electrolyte foils 11 , 12 , 13 , which are made of, for example, Y 2 O 3 -stabilized ZrO 2 .
- An outer measuring electrode 15 which is covered by a porous protective coating 16 , is arranged on the outer large surface of first foil 11 .
- Protective coating 16 is made of, for example, porous ZrO 2 or Al 2 O 3 .
- a reference channel 17 is provided in second foil 12 and is connected to a reference atmosphere, e.g., air.
- a reference electrode 18 which is arranged on first foil 11 and faces measuring electrode 15 , is arranged in reference channel 17 .
- a heating device 22 is integrated into basic body 10 , and on third foil 13 electrical insulating layers 21 are provided, in which heating device 22 is embedded. Heating device 22 is an electrical-resistor-type heating element.
- the layer structure of measuring electrode 15 is as shown in FIG. 2.
- an electrically conductive base layer 25 which is made of, for example, a Pt cermet, is arranged on foil 11 of basic body 10 .
- Protective coating 16 is provided on base layer 25 .
- further layer 27 is formed in the pores of protective coating 16 and is adjacent to and on top of base layer 25 . This layer 27 is directly in contact with base layer 25 .
- Base layer 25 and further layer 27 form measuring electrode 15 . We will discuss how layer 27 is manufactured below.
- layer 27 may be made of a material that inhibits, i.e., impedes, establishment of equilibrium in the gas mixture on the surface of the electrode.
- materials include, for example, precious metals (gold, rhodium, iridium), semiprecious metals (palladium, silver), base metals (copper, bismuth, nickel, chrome) or a mixture of these metals.
- further layer 27 is made of gold.
- Mixed potential electrodes are electrodes that cannot or can only incompletely catalyze the establishment of equilibrium in the gas mixture.
- measuring electrode 15 along with reference electrode 18 , which is made of, for example, Pt and is arranged in reference channel 17 , form a mixed potential sensor.
- the material of layer 27 of measuring electrode 15 which does not or only incompletely catalyzes establishment of equilibrium in the gas mixture, causes a competing reaction between the oxygen and the oxidizable gas components to occur at measuring electrode 15 . Accordingly, very little of the CO conveyed along with the measured gas reacts with the free oxygen to form CO 2 . As a result, free oxygen as well as CO reach the three-phase boundary of measuring electrode 15 and contribute to the signal generated there.
- measuring electrode 15 and reference electrode 18 A potential difference arises between measuring electrode 15 and reference electrode 18 , where constant oxygen partial pressure is present thanks to the reference air, and can be detected as an electromotive force by a measuring instrument 30 .
- the electromotive force is therefore dependent on the oxidizable gas components.
- one can, for example, improve the behavior of an oxygen sensor at low temperatures by using an Rh layer on a Pt electrode.
- a second exemplary embodiment of a layer system for measuring electrode 15 is shown in FIG. 3.
- layer 27 is created in the pores of protective coating 16 , and on top of layer 27 a second layer 28 is created, and on top of layer 28 a third layer 29 is created.
- layer 27 is made of, for example, gold
- layer 28 is made of, for example, rhodium or iridium
- layer 29 is made of nickel or chrome.
- This exemplary embodiment shows that it is easy to achieve a complex, multi-layer electrode structure.
- By using the layer structure shown in FIG. 3 and/or choosing an appropriate material for layers 27 , 28 , 29 one can modify, for example, the catalytic characteristics of the electrode in a specific manner.
- the sensor element basic body 10 described.
- the appropriate functional layers are applied to foils 11 , 12 , 13 , these being in their green (unvitrified) state.
- a Pt-cermet paste is applied to the large surface of first foil 11 to create base layer 25
- a Pt-cermet paste is also applied to its other large surface to create reference electrode 18 .
- Protective coating 16 is, for example, screen-printed or painted on top of the Pt-cermet paste of base layer 25 on the large surface of foil 11 .
- the material of protective coating 16 contains pore-formers which vaporize and, respectively, combust during the subsequent vitrification process so as to form pores.
- Insulating layers 21 are applied to foil 13 via screen-printing steps, and heating device 22 is arranged between insulating layers 21 .
- Foils 11 and 13 to which the functional layers have been applied, are laminated with foil 12 , into which reference channel 17 has first been punched, and vitrified at a temperature of, for example, 1400° C.
- basic body 10 Following vitrification, basic body 10 is present, its structure matching that of a sensor element of an oxygen sensor for determining the lambda value in gas mixtures.
- layer 27 according to FIG. 2 or a plurality of layers 27 , 28 , 29 according to FIG. 3 are applied to basic body 10 , which is in its vitrified state, layer 27 and, respectively, layers 27 , 28 , 29 being formed in layer levels in the pores of porous protective coating 16 .
- Layers 27 , 28 , 29 are manufactured via galvanic deposition. To accomplish this, the ceramic body is placed in a galvanizing bath. Base layer 25 is electrically connected as the cathode, the connection contact point of base layer 13 , which is present on sensor element basic body 10 , being used as the contact point. As the anode, a metal, for example, is immersed in the galvanizing bath, this metal corresponding to the metal of respective layer 27 , 28 , 29 to be deposited (galvanizing method using a sacrificial anode). For example, water-soluble, ionic salts of the metal in question, e.g., HAuCl 4 , IrC 3 x H 2 O or RhCl 3 x H 2 O, are used as the electrolyte.
- HAuCl 4 e.g., IrC 3 x H 2 O or RhCl 3 x H 2 O
- a layer system according to FIG. 2 is selected, further layer 27 in the form of a gold layer being deposited on base layer 25 , which is made of Pt-cermet, via galvanic deposition.
- vitrified basic body 10 is, for example, placed in a galvanizing bath containing an HAuCl 4 electrolyte, a gold anode being used. If a current of 0.5 to 2 mA is applied for 15-50 minutes, gold layer 27 having a thickness of, for example, 1-5 ⁇ m, is deposited on Pt-cermet base layer 25 .
- layer 27 forms in the pores of protective coating 16 .
- the ceramic body is subjected to an annealing process at a temperature of, for example, 1200° C.
- an alloy of the Pt of base layer 25 and the gold of layer 27 namely a platinum-rich gold phase and a gold-rich platinum phase, is formed.
- the catalytic activity of the Pt of Pt-cermet base layer 25 is modified, and a mixed potential electrode is created as measuring electrode 15 , this being selective with respect to hydrocarbons.
- the layer system according to FIG. 3 is also manufactured via galvanic deposition, the appropriate anode materials and/or the appropriate galvanizing baths being used in sequence during galvanic deposition.
- the appropriate anode materials and/or the appropriate galvanizing baths being used in sequence during galvanic deposition.
- further combinations and layer systems for electrodes of gas sensors are conceivable, these being deposited as a porous layer on an electrically conductive base layer.
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Abstract
A gas sensor and a method for its manufacture are described. The gas sensor has a solid electrolyte (11) having at least one measuring electrode (15) and one porous protective coating (16). The measuring electrode (15) has an electrically conductive base layer (25) and a further layer (27), the further layer (27) being deposited in the pores of the porous protective coating (16) adjacent to the base layer (25) via galvanic deposition. In order to deposit the further layer (27) via galvanic deposition, the basic body (10), which has been fused with the base layer (25) and the protective coating (16) via vitrification, is immersed in a galvanizing bath, the base layer (25) being connected as the cathode.
Description
- The invention relates to a gas sensor according to the preamble of
claim 1 and a method for manufacturing the gas sensor. - From German Offenlegungsschrift 23 04 464 a probe is known in which a gold or silver electrode which does not catalyze establishment of equilibrium in the gas mixture and works in conjunction with a platinum electrode that does catalyze establishment of equilibrium in the measured gas is provided. The catalytically inactive electrode materials cause a competing reaction between the oxygen and the oxidizable and, respectively, reducible gas components to take place at that electrode. Even if adjustments have been made to ensure high lambda values, very little of the free oxygen that is conveyed along with the measured gas reacts with, for example, C3H6 or CO; as a result, free oxygen as well as C3H6 and, respectively, CO reach the three-phase boundary at the catalytically inactive electrode (non-equilibrium state).
- A gas sensor having a measuring electrode and a reference electrode arranged on a solid electrolyte is known from European Patent 466 020 A. In order to create a mixed potential electrode, the measuring electrode is made of a platinum compound or a ternary alloy that includes platinum, gold, nickel, copper, rhodium, ruthenium, palladium or titanium. Herein, the materials may be applied to the solid electrolyte as multiple layers, the alloying step being carried out after the materials are applied.
- From U.S. Pat. No. 4,199,425, a gas sensor in which a platinum electrode covered by a porous protective coating is provided is additionally known. The pores of the protective coating are impregnated with a further catalytic material, rhodium. The rhodium renders the gas sensor sensitive to NOx, as well as oxygen. Herein, the rhodium coats the walls of the pores of the entire protective coating; as a result it is impossible to specify the thickness of the layer in the porous protective coating.
- The gas sensor according to the present invention having the characterizing features set forth in
claim 1 has the following advantage: a vitrified sensor element basic body can be used, the further layer being integrated via just one additional deposition step following vitrification. As a result, the outer electrode of the sensor element basic body can be modified following vitrification. The sensor element of a Nernst-type lambda sensor, for example, can be used as the sensor element basic body, it being possible to transform the outer electrode into a mixed potential electrode by making certain modifications. Furthermore, it is advantageous that materials that would not withstand the high temperature at which vitrification is carried out can be used as the further layers. A further advantage is that the further layer system, which is directly adjacent to the electrically conductive base layer, does not completely fill the pores of the porous protective coating. As a result, the porous protective coating continues to provide effective protection, and sufficient gas can access the three-phase boundary. Herein, the material used as the further layer may be used to modify the functional characteristics of the electrode of the gas sensor in a specific manner. Herein, this modification may define the specific gas selectivity of the sensor and/or its position within the control system. - Advantageous further refinements of the gas sensor according to the present invention and the method according to the present invention can be achieved via the measures set forth in the subordinate claims. A particularly advantageous sensor designed for mixed potentials can be achieved if the layer system is subjected to a thermal additional treatment following deposition of the further layer. For example, in the case of a Pt/Au electrode a temperature range of 1200° C. ±100° C. is favorable. At this temperature, the metal atoms of the further layer diffuse into the metal of the adjacent base layer. A further advantage is that a cermet layer is used as the electrically conductive base layer which, thanks to its ceramic component, creates a solid join with the solid electrolyte when the ceramic body is vitrified. Furthermore, by creating a plurality of further layers and choosing the layer material appropriately one can specify the selectivity and also modify the catalytic activity of the electrode with even greater precision.
- Exemplary embodiments of the present invention are shown in the drawing and described in greater detail in the description below.
- FIG. 1 shows a section through a gas sensor according to the present invention;
- FIG. 2 shows an enlarged sectional view of a first exemplary embodiment of an electrode of the gas sensor according to the present invention; and
- FIG. 3 shows an enlarged sectional view of a second exemplary embodiment of an electrode of the gas sensor according to the present invention.
- FIG. 1 shows a gas sensor having a sensor element
basic body 10 whose structure corresponds to that of a Nernst-type oxygen sensor (lambda sensor).Basic body 10 includes, for example, a plurality of ceramicsolid electrolyte foils outer measuring electrode 15, which is covered by a porousprotective coating 16, is arranged on the outer large surface offirst foil 11.Protective coating 16 is made of, for example, porous ZrO2 or Al2O3.A reference channel 17 is provided insecond foil 12 and is connected to a reference atmosphere, e.g., air. Areference electrode 18, which is arranged onfirst foil 11 andfaces measuring electrode 15, is arranged inreference channel 17. Aheating device 22 is integrated intobasic body 10, and onthird foil 13electrical insulating layers 21 are provided, in whichheating device 22 is embedded.Heating device 22 is an electrical-resistor-type heating element. - According to a first exemplary embodiment, the layer structure of measuring
electrode 15 is as shown in FIG. 2. According to this structure, an electricallyconductive base layer 25, which is made of, for example, a Pt cermet, is arranged onfoil 11 ofbasic body 10.Protective coating 16 is provided onbase layer 25. According to FIG. 2,further layer 27 is formed in the pores ofprotective coating 16 and is adjacent to and on top ofbase layer 25. Thislayer 27 is directly in contact withbase layer 25.Base layer 25 andfurther layer 27form measuring electrode 15. We will discuss howlayer 27 is manufactured below. - Herein,
layer 27 may be made of a material that inhibits, i.e., impedes, establishment of equilibrium in the gas mixture on the surface of the electrode. Such materials include, for example, precious metals (gold, rhodium, iridium), semiprecious metals (palladium, silver), base metals (copper, bismuth, nickel, chrome) or a mixture of these metals. In the present exemplary embodiment according to FIG. 2,further layer 27 is made of gold. As a result, measuringelectrode 15 of the sensor shown in FIG. 1 is transformed into a mixed potential electrode that is selective with respect to hydrocarbons (HC). - Mixed potential electrodes are electrodes that cannot or can only incompletely catalyze the establishment of equilibrium in the gas mixture. Herein, measuring
electrode 15, along withreference electrode 18, which is made of, for example, Pt and is arranged inreference channel 17, form a mixed potential sensor. The material oflayer 27 of measuringelectrode 15, which does not or only incompletely catalyzes establishment of equilibrium in the gas mixture, causes a competing reaction between the oxygen and the oxidizable gas components to occur at measuringelectrode 15. Accordingly, very little of the CO conveyed along with the measured gas reacts with the free oxygen to form CO2. As a result, free oxygen as well as CO reach the three-phase boundary of measuringelectrode 15 and contribute to the signal generated there. A potential difference arises between measuringelectrode 15 andreference electrode 18, where constant oxygen partial pressure is present thanks to the reference air, and can be detected as an electromotive force by ameasuring instrument 30. The electromotive force is therefore dependent on the oxidizable gas components. Thus one can specify the selectivity of measuringelectrode 15 to a given gas type by choosing the material used forfurther layer 27 appropriately, so that it is possible to minimize the extent to which it is cross-sensitive to other gas components. Furthermore, one can, for example, improve the behavior of an oxygen sensor at low temperatures by using an Rh layer on a Pt electrode. - A second exemplary embodiment of a layer system for measuring
electrode 15 is shown in FIG. 3. On top ofbase layer 25,layer 27 is created in the pores ofprotective coating 16, and on top of layer 27 asecond layer 28 is created, and on top of layer 28 athird layer 29 is created. In the exemplary embodiment shown in FIG. 3,layer 27 is made of, for example, gold;layer 28 is made of, for example, rhodium or iridium; andlayer 29 is made of nickel or chrome. This exemplary embodiment shows that it is easy to achieve a complex, multi-layer electrode structure. By using the layer structure shown in FIG. 3 and/or choosing an appropriate material forlayers - To manufacture the sensor according to FIG. 1, one uses, for example, the sensor element
basic body 10 described. The appropriate functional layers are applied to foils 11, 12, 13, these being in their green (unvitrified) state. Herein, a Pt-cermet paste is applied to the large surface offirst foil 11 to createbase layer 25, and a Pt-cermet paste is also applied to its other large surface to createreference electrode 18.Protective coating 16 is, for example, screen-printed or painted on top of the Pt-cermet paste ofbase layer 25 on the large surface offoil 11. Herein, the material ofprotective coating 16 contains pore-formers which vaporize and, respectively, combust during the subsequent vitrification process so as to form pores. Insulatinglayers 21 are applied to foil 13 via screen-printing steps, andheating device 22 is arranged between insulatinglayers 21.Foils foil 12, into whichreference channel 17 has first been punched, and vitrified at a temperature of, for example, 1400° C. - Following vitrification,
basic body 10 is present, its structure matching that of a sensor element of an oxygen sensor for determining the lambda value in gas mixtures. In the case of the present exemplary embodiments,layer 27 according to FIG. 2 or a plurality oflayers basic body 10, which is in its vitrified state,layer 27 and, respectively, layers 27, 28, 29 being formed in layer levels in the pores of porousprotective coating 16. - Layers27, 28, 29 are manufactured via galvanic deposition. To accomplish this, the ceramic body is placed in a galvanizing bath.
Base layer 25 is electrically connected as the cathode, the connection contact point ofbase layer 13, which is present on sensor elementbasic body 10, being used as the contact point. As the anode, a metal, for example, is immersed in the galvanizing bath, this metal corresponding to the metal ofrespective layer - In order to manufacture a sensor for measuring hydrocarbons, a layer system according to FIG. 2 is selected,
further layer 27 in the form of a gold layer being deposited onbase layer 25, which is made of Pt-cermet, via galvanic deposition. To accomplish this, vitrifiedbasic body 10 is, for example, placed in a galvanizing bath containing an HAuCl4 electrolyte, a gold anode being used. If a current of 0.5 to 2 mA is applied for 15-50 minutes,gold layer 27 having a thickness of, for example, 1-5 μm, is deposited on Pt-cermet base layer 25. Herein,layer 27 forms in the pores ofprotective coating 16. Afterlayer 27 has been deposited, the ceramic body is subjected to an annealing process at a temperature of, for example, 1200° C. During the annealing process, an alloy of the Pt ofbase layer 25 and the gold oflayer 27, namely a platinum-rich gold phase and a gold-rich platinum phase, is formed. As a result, the catalytic activity of the Pt of Pt-cermet base layer 25 is modified, and a mixed potential electrode is created as measuringelectrode 15, this being selective with respect to hydrocarbons. - The layer system according to FIG. 3 is also manufactured via galvanic deposition, the appropriate anode materials and/or the appropriate galvanizing baths being used in sequence during galvanic deposition. In addition to the layer systems shown in FIGS. 2 and 3 and described above, further combinations and layer systems for electrodes of gas sensors are conceivable, these being deposited as a porous layer on an electrically conductive base layer.
Claims (11)
1. A gas sensor having a solid electrolyte, having at least one measuring electrode arranged on the solid electrolyte and having a porous protective coating, which is arranged on top of the measuring electrode, characterized in that the measuring electrode (15) has an electrically conductive base layer (25) and at least one further layer (27, 28, 29), the further layer (27, 28, 29) being formed in the pores of the porous protective coating (16) adjacent to the base layer (25).
2. The gas sensor according to claim 1 , characterized in that the further layer (27, 28, 29) contains at least one material that modifies the functional characteristics of the base layer (25) by forming an alloy with the material of the base layer (25).
3. The gas sensor according to claim 1 or 2,
characterized in that the further layer (27, 28, 29) is made of precious metals, semiprecious metals, base metals or a mixture of these metals.
4. The gas sensor according to claim 1 , 2 or 3,
characterized in that a plurality of further layers (27, 28, 29) are applied to the base layer (25) in any order.
5. The gas sensor according to claim 1 ,
characterized in that the base layer (25) is a cermet layer.
6. The gas sensor according to claim 5 ,
characterized in that the base layer (25) is a Pt-cermet layer.
7. A method for manufacturing a gas sensor having an electrically conductive base layer arranged on a solid electrolyte and a porous protective coating arranged over the base layer, at least the solid electrolyte and the base layer being vitrified to form a ceramic basic body, characterized in that after the vitrification process at least one further layer is deposited on the base layer through the porous protective coating via galvanic deposition.
8. The method according to claim 7 ,
characterized in that the further layer is deposited via cathodic deposition.
9. The method according to claim 7 ,
characterized in that the vitrified basic body is placed in a galvanizing bath, the base layer is connected as the cathode using the connection contact points that are already present on the basic body, and a metal that corresponds to the material of the further layer is used as the anode.
10. The method according to claim 7 ,
characterized in that after the further layer is deposited via galvanic deposition, the layer system is subjected to a further heat treatment.
11. The method according to claim 10 ,
characterized in that the temperature reached during the heat treatment is lower than that reached when the ceramic body is vitrified.
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
DE19833087A DE19833087A1 (en) | 1998-07-23 | 1998-07-23 | Gas sensor for vehicle engine; has measuring electrode comprising platinum base with sintered porous layer and noble metal covering layer, applied in galvanic bath |
DE19833087 | 1998-07-23 | ||
DE19833087.1 | 1998-07-23 | ||
PCT/DE1999/001727 WO2000005573A1 (en) | 1998-07-23 | 1999-06-12 | Gas sensor and corresponding production method |
Publications (2)
Publication Number | Publication Date |
---|---|
US20020023838A1 true US20020023838A1 (en) | 2002-02-28 |
US6395161B1 US6395161B1 (en) | 2002-05-28 |
Family
ID=7874996
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US09/509,270 Expired - Fee Related US6395161B1 (en) | 1998-07-23 | 1999-06-12 | Gas sensor and corresponding production method |
Country Status (5)
Country | Link |
---|---|
US (1) | US6395161B1 (en) |
EP (1) | EP1040346B1 (en) |
JP (1) | JP2002521662A (en) |
DE (2) | DE19833087A1 (en) |
WO (1) | WO2000005573A1 (en) |
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1999
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- 1999-06-12 JP JP2000561489A patent/JP2002521662A/en active Pending
- 1999-06-12 WO PCT/DE1999/001727 patent/WO2000005573A1/en active IP Right Grant
- 1999-06-12 US US09/509,270 patent/US6395161B1/en not_active Expired - Fee Related
- 1999-06-12 DE DE59914732T patent/DE59914732D1/en not_active Expired - Lifetime
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Also Published As
Publication number | Publication date |
---|---|
DE59914732D1 (en) | 2008-05-29 |
WO2000005573A1 (en) | 2000-02-03 |
US6395161B1 (en) | 2002-05-28 |
EP1040346A1 (en) | 2000-10-04 |
EP1040346B1 (en) | 2008-04-16 |
DE19833087A1 (en) | 2000-01-27 |
JP2002521662A (en) | 2002-07-16 |
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