Classifications

• • 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
  • G01N27/4065—Circuit arrangements specially adapted therefor
• • 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
  • G01N27/4073—Composition or fabrication of the solid electrolyte
  • G01N27/4074—Composition or fabrication of the solid electrolyte for detection of gases other than oxygen

Definitions

• the invention relates to a method for measuring the concentration of at least one predetermined gas in a gas mixture by means of an electrolyte provided with first and second electrodes, which is exposed to the gas mixture together with the electrodes connected to a voltage source, the voltage source being one caused by electrodes and electrolytes flowing electrical current, which is dependent on the ion concentration and is measured as a signal of the gas concentration, and an electrochemical sensor.
• the method proves to be problematic with regard to its relatively complex structure for generating the thermoelectric potential and possibly temperature control.
• EP-OS 0 019 731 discloses a polarographic sensor for determining the oxygen content in gases, in particular in exhaust gases from internal combustion engines, which has an oxygen-ion-conducting solid electrolyte body which is provided with an anode and a cathode to which a constant voltage is to be applied ; the cathode is covered by a layer containing pores or channels as a diffusion barrier and provided with a reference to the setting of the thermodynamic gas equilibrium of catalytically active material, while the anode consists of a catalytically inactive material; Both electrodes are exposed to the gas to be measured when the sensor operates according to the diffusion limit current principle.
• US Pat. No. 5,344,549 discloses a method for determining the partial oxygen pressure and an partial oxygen pressure sensor; a solid electrolyte with variable electron conductivity is provided with two electrodes, of which only one electrode, which is switched off in porous form, is exposed to the gas to be measured, while the other electrode, which is known as the blocking electrode, is not exposed to the exhaust gas to be measured. A flow of oxygen ions within the solid electrolyte is not provided.
• an electrochemical sensor for the determination of the oxygen content in gases, in particular in exhaust gases from internal combustion engines with a solid electrolyte and at least one electrode as a sensor is known, which is to be measured on the Gas-exposed side of the solid electrolyte is arranged, wherein the sensor contains a porous ceramic protective layer made of an aluminum oxide and / or a magnesium spinel matrix with zirconium dioxide particles embedded therein.
• a catalytically active material particles of platinum or a platinum alloy are added to the matrix of the protective layer.
• One electrode is exposed to the gas mixture to be measured, while the other is exposed to an additional reference gas.
• An oxygen sensor for monitoring the oxygen content of exhaust gases from internal combustion engines is known from US Pat. No. 4,221,650, which has a solid electrolyte with oxygen-ion-conducting.
• a solid electrolyte with oxygen-ion-conducting Contains zirconium dioxide, which is dispersed with 15 to 50% by volume in an oxide composite contained in aluminum oxide, the solid electrolyte being in contact with electrodes spaced from one another; the sensor is designed as a cylindrical tube closed on one side, zirconium dioxide being stabilized with yttrium oxide in the rounded, closed end.
• the electrodes are applied on both sides of the solid electrolyte, the outer surface of the sensor being provided with a porous insulating layer, such as magnesium spinel.
• US Pat. No. 3,960,693 describes an electrochemical device for measuring the oxygen concentration in exhaust gases, in particular those from internal combustion engines, a tubular solid electrolyte with a passage consisting of two spaced open ends, an inner circumferential shoulder being arranged between these ends ; a tubular part made of an ion-conducting solid electrolyte has a closed first region which protrudes from one of the open ends and a second region which extends through the passage passage and is provided with an outer circumferential flange; both on its outer surface and on its inner surface, the tubular part is provided with an electron-conducting catalytic layer, which can consist, for example, of platinum; the outer layer applied, which is exposed to the ambient atmosphere, serves as a measuring electrode.
• a tubular solid electrolyte with a passage consisting of two spaced open ends, an inner circumferential shoulder being arranged between these ends ; a tubular part made of an ion-conducting solid electrolyte has a closed first region which pro
• EP 0 294 085 B1 discloses an electrochemical element with a solid electrolyte b, which consists of a dense solid electrolyte body and a porous, solid electrolyte, the cermet electrode applied in the outer region of the closed end preferably by mixing a powder of a platinum group metal such as platinum, rhodium, palladium, iridium, ruthenium or osmium or a metal such as gold or nickel with a ceramic powder such as zirconium oxide, hydrium oxide or aluminum oxide in such a way that the metallic powder is not less than 40% by volume.
• an electrode is also brought up as a reference electrode, which is exposed to the air as a reference gas. It is thus possible to determine the oxygen potential pressure in the gas to be measured by means of the electromotive force that is generated between the first electrode and the second electrode.
• diffusion holes or diffusion layers which can be contaminated when used in exhaust gases, proves to be problematic; furthermore, reference measurements by means of reference gas or reference electrode lead to relatively complex measuring arrangements or measuring methods.
• the object of the invention is to determine the concentration of at least one gas in a gas mixture by means of an electrochemical cell with electrodes z subjected to voltage, reference measurements, e.g. should be dispensed with by means of reference gas or reference electrode. Diffusion holes or diffusion layers, which can be contaminated when exhaust gases from internal combustion engines are used, should also be avoided. Furthermore, the concentration of different gases should be made possible by varying the applied voltage and / or their polarity and by using different electrode materials.
• the object is achieved with respect to a method for measuring the concentration by the characterizing features of claim 1. It has proven to be particularly advantageous that the concentration of a gas in one measurement, or the concentration of several gases in the case of several measurements, can be carried out in a relatively simple manner.
• planar type of sensors based on the present invention is to be regarded as particularly advantageous.
• Such a planar configuration enables the production of an arrangement of several sensors in a single work step.
• Another advantage is the possibility of a temperature-dependent evaluation of the gas concentration with the aid of temperature control of the sensor.
• rhenium is particularly advantageous with regard to its specific catalytic activity in the detection of hydrocarbons.
• FIG. 1 shows the basic principle of the physical mode of operation of the present invention
• FIG. 2 shows a practical embodiment of the sensor with contacting of the electrolyte by applied electrodes
• FIG. 3 shows a possible embodiment of the sensor in planar construction according to the present invention.
• FIG. 4a shows in longitudinal section a sensor housing for a sensor according to FIG. 3
• Figure 4b shows a cross section along the line AB of Figure 4a.
• FIG. 5 shows characteristic curves of the current as a function of the applied voltage for various oxygen concentrations at a temperature of 900 ° C. in a characteristic curve diagram, zirconium dioxide stabilized with yttrium oxide being used as the electrolyte.
• FIG. 6 shows characteristic curves of a differential current as a function of the oxygen concentration for different voltages applied in the case of a zirconium dioxide electrolyte stabilized with yttrium oxide, the differential current being obtained by reversing the polarity of a pair of electrodes from planar gold and platinum electrodes ; the temperature is 900 ° C
• Figure 7 shows the design of a sensor with more than two electrodes, which is based on the vorlie invention.
• the senor 2 according to the invention is exposed to the gas phase 1; it has an electrolyte 3, which is preferably designed as a solid electrolyte.
• the electrolyte 3 is in electrical contact with a first electrode 4 and a second electrode 5, the first electrode being designed as a catalytically active element.
• the first electrode 4 consists at least on its surface of platinum or a platinum group metal.
• the second Elektro de 5 is made of gold.
• the electrolyte 3 consists of zirconium dioxide stabilized with yttrium.
• the electrodes 4, 5 of the sensor 2 are connected via connections 6, 7 to a series connection of a voltage source 8 and an ammeter 9.
• the electrolyte is in contact with the electrodes 4, 5, which at least consist of different materials on their contacting surface.
• the first electrode is preferably made of platinum, the second electrode of gold; the electrolyte speaks to the electrolyte known from FIG.
• the electrodes 4, 5 and the electrolyte 3 are arranged in an isothermal area 24, the electrodes 4, 5 leading out via the lead 6, 7, out of the thermally insulated area 24 and contacted to the outside; it is very important that the leads 6, 7 do not come into contact with the electrolyte; the connecting conductors 6, 7 preferably consist of the same material, for example made of platinum for both connecting conductors, so that thermal voltage can be avoided due to the isothermal region 24.
• the electrodes 4 and 5 and the solid electrolyte 3 are typically exposed to the gas 1 to be analyzed or the gas phase to measure the gas phase 1; during the measurement process one or more of the gas components are adsorbed and desorbed on the negative electrode 4; the first possibility is that a gas molecule adsorbs on the electrode material; the adsorbed molecule is then broken down into individual atoms as it is adsorbed onto the electrode material. The adsorbed molecules or the adsorbed atoms consequently migrate to the contact area between the electrode 4, the gas phase 1 and the solid electrolyte 3. Such a contact area where three phases meet is referred to as a triple point; it is provided with reference number 12 here. The line formed by triple points is called the triple line.
• gaseous components adsorb directly to the triple point.
• one of these gaseous components adsorbed can be the gas to be measured.
• the adsorbed molecules or adsorbed atoms there are two possibilities for the adsorbed molecules or adsorbed atoms; a first possibility is that they are converted into anions by taking up electrons which are present in the electrode material.
• the electrolyte 3 should be a conductor for these anions and the gas to be measured should be composed of molecules which correspond to the anions.
• the material of the electrode 4 is chosen so that this reaction is promoted; this means that the material of the electrode 4 serves as a catalyst for the ionization reaction.
• a typical example can be seen in the fact that oxygen molecules are converted into oxygen atoms and these oxygen atoms are converted to O 2 ions at the triple points 12 by taking up two electrons from electrode 4.
• Electrolyte zirconia As a suitable electrolyte zirconia is to be regarded as Zirkon ⁇ dioxide a good conductor of O 'ions at high temperatures. Platinum has proven to be a suitable material for the first electrode 4, while the second electrode 5 consists of gold.
• the second possibility is for a reaction between two or more substances to occur at the triple points. At least one of the substances should be in adsorbed form and at least one of the substances should be the gas to be measured. If an atom of type X is exchanged between the reacting substances in this reaction, then the Electrolyte to be a conductor for type X ions.
• An example of such a reaction is set out below:
• the electrolyte should therefore be an oxygen ion conductor, as is the case for zirconium dioxide.
• the type X ion is an oxygen ion.
• the concentration of these oxygen ions in the electrolyte near the triple point is important because it affects the balance between the reactants.
• the material of the electrode 4 is selected in order to accelerate the reaction. This means that the materi al of the electrode 4 acts as a catalyst for the reaction.
• the number of anions that are led from electrode 4 through electrolyte 3 to electrode 5 depends on the concentration of the anions that are available at electrode 4. This concentration is determined by the chemical reaction in the vicinity of the triple point of electrode 4 and consequently by the concentration of the gaseous substance which is to be determined.
• the ion flow rate in the electrolyte is influenced by various factors.
• One of these factors is the extent of the catalysis of the reaction by the material of the electrode 4; another factor is the concentration of the gas to be measured.
• Other factors include the voltage of the voltage source 8, the temperature, the size of the surface of the electrodes 4 and 5. Since the ion flow rate through the electrolyte is directly proportional to the electrical current measured by ammeter 9, this current is also influenced by the concentration of the gas to be measured.
• the rate determining the rate of adsorption, reaction, migration and desorption can also be connected to electrode 5 instead of electrode 4, as described in the previous paragraphs.
• the electrical current I is determined by the catalytic action of the electrode 5.
• the electrode material of the electrode 5 is very important in this case, more important than the material of the electrode 4.
• the relative sizes de Electrodes 4 and 5 also determine whether the rate determining step occurs on electrode 4 or electrode 5.
• the materials of the electrodes 4 and 5 have to be different. In the event that different materials are selected for the electrodes, it is possible to increase or decrease the sensitivity of the sensor by reversing the polarity of the electrodes.
• the current I is measured using an ammeter 9 using the electrical polarity according to FIG. 1, this means that electrode 4 is connected to the negative pole and electrode 5 to the positive pole of the electrical voltage source 8. If the ions of the electrolyte are negative ions, an ion current will flow according to the ion transport arrow 11 in FIG. 1. It is assumed that the rate-determining step for the formation of ions and for the desorption reaction, as mentioned above, is bound to electrode 4.
• the catalytic activity of the electrode 4 determines the strength of the electrical current I.
• the polarity of the electrical voltage is reversed, a different current I will flow, since the electrode 5 is made of a different material than the electrode 4.
• the materials of the electrodes 4 and 5 it is possible to make the difference between the two measured currents as high as possible, which increases the sensitivity of the gas to be measured. In most cases, high sensitivity is desirable. In the event that the range of the gas concentration to be measured is very high, it is desirable to have different ranges of sensitivity available. This can also be realized with different electrode materials, each of which results in a specific sensitivity of the gas to be measured.
• FIG. 3 is an example of a planar oxygen sensor.
• a planar electrolyte 3 is applied to an inert substrate 10; the electrolyte 3 consists of zirconium dioxide stabilized with yttrium oxide or magnesium oxide, the substrate of aluminum oxide.
• the electrolyte 3 is covered by planar electrodes 4, 5.
• Electrode 4 is made of platinum and electrode 5 is made of gold. In this case, the length of the triple line is as large as possible in order to increase the ion flow through the electrolyte, which results in a high sensitivity.
• FIG. 4a shows an exemplary practical embodiment for a sensor according to FIG. 3, which is aimed at high-temperature applications, for example in exhaust gas systems of internal combustion engines.
• the substrate 10 of the sensor is arranged in the head region 13 of a metal housing 15.
• the head portion 13 is in a threaded opening screwed in.
• the head region 13 consists of a heat-resistant material.
• the connections 4, 5 of the electrodes are contacted by means of connection wires 6, 7 (see also FIGS. 3 and 2).
• the connecting wires 6, 7 are pressed onto the sensor substrate by means of the heat-resistant and electrically insulating pressing body 25.
• the press body 25 is made of aluminum oxide, but it can also consist of cordierite or dichroic. Press body 25 is pressed onto the connecting wires 6, 7 by means of the heat-resistant spring 16.
• the metal housing 1 consists of heat-resistant metal and is welded to the head region 13 in the edge region 1; the welding is gastight. In order to prevent gas leaks via the connecting wires 6, a heat-resistant seal 19 is used.
• the characteristic curve diagram according to FIG. 5 is directed to an embodiment in which the sensitivity is increased by using two different electrode materials;
• the embodiment is directed to a sensor in which zirconium dioxide stabilized with yttrium oxide is used as the electrolyte, the gas to be measured is oxygen and the electrodes 5 and 5 each consist of platinum and gold.
• the temperature of the gas and the sensor is 900 ° C.
• a polarity reversal of the electrodes determines a difference between the current at which the platinum electrode is switched as a positive pole and the current at which the gold electrode is switched as a positive pole, which results in a higher sensitivity with regard to the oxygen content than in the case where the polarity is not changed.
• curves A, B and C apply to a positively switched platinum electrode, with an oxygen content of 0% for curve A, an oxygen content of 1% for curve B and an oxygen content of 20% for curve C; it can be seen that with an applied voltage of 6V according to curve A, a current of approx. 1.90 mA, according to curve B a current of approx. 2.4 mA and according to curve C a current of approx.
• FIG. 6 shows as a characteristic diagram the difference between the two currents I for a positive platinum electrode and a positive gold electrode as a function of the applied oxygen concentration c, which is derived directly from the experimental result of FIG. 5 ⁇ can be directed.
• the sensitivity to oxygen also depends on the level of the electrical voltage.
• FIG. 7 shows an embodiment of a sensor with more than two electrodes, in which the method according to the invention is applied.
• the sensor is planar.
• the electrolyte 3 is covered with three electrodes 4, 5 and 20; the electrodes 4, 5, 20 are made of similar or different types of materials, but this depends on the gas to be determined.
• electrode 4 consists, for example, of platinum, electrode 5 of gold and electrode 20 of platinum; zirconium dioxide stabilized with yttrium oxide or magnesium oxide is used as the electrolyte 3.
• the first current to be measured flows through the electrodes 4 and 5; the second current I 'to be measured flows through the electrodes 4 and 20.
• FIG. 1 shows an embodiment of a sensor with more than two electrodes, in which the method according to the invention is applied.
• the sensor is planar.
• the electrolyte 3 is covered with three electrodes 4, 5 and 20; the electrodes 4, 5, 20 are made of similar or different types of materials, but this depends on the gas to be determined.
• electrode 4 consists

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• 1994
  • 1994-12-16 DE DE4445033A patent/DE4445033A1/de not_active Withdrawn
• 1995
  • 1995-12-12 JP JP8518254A patent/JPH10510622A/ja not_active Ceased
  • 1995-12-12 US US08/849,573 patent/US5820745A/en not_active Expired - Fee Related
  • 1995-12-12 WO PCT/EP1995/004899 patent/WO1996018890A1/de not_active Application Discontinuation
  • 1995-12-12 RU RU97111884A patent/RU2143679C1/ru active
  • 1995-12-12 CN CN95196804.1A patent/CN1050667C/zh not_active Expired - Fee Related
  • 1995-12-12 BR BR9510040A patent/BR9510040A/pt active Search and Examination
  • 1995-12-12 EP EP95942123A patent/EP0797770A1/de not_active Withdrawn

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