Marginal current detector for measuring the partial pressure of a gas.

0001The invention relates to a limit current sensor for measuring the oxygen gas partial pressure, with an ion-conducting zirconia solid electrolyte element, the electrolyte path of which is arranged between gas-permeable electrodes connected to a direct voltage source and with a diffusion barrier which limits gas access to the cathode side of the electrolyte path.

0002Limit current sensors constructed using cubic stabilized zirconium dioxide (ZrO₂) for measuring the oxygen gas partial pressure are known (H. Jahnke, B. Moro, H. Dietz and B. Beyer, Ber. Bunsenges. Phys. Chem. 92, 1250-1257 (1988 ) and H. Dietz, Solid State Ionics 6 (1982), 175-183). With these limit current sensors, the oxygen access to the ZrO₂ solid electrolyte is limited by a diffusion barrier on the cathode side of the electrolyte path. ZrO₂ is a pure oxygen ion conductor, which "pumps" oxygen through the electrolyte element due to the potential difference between the electrodes of the electrolyte path. Since the diffusion barrier limits the oxygen supply on the cathode side of the electrolyte path, a state of equilibrium is established between the oxygen molecules supplied through the diffusion barrier and the oxygen molecules "pumped out" via the solid ion element conducting oxygen ions. If the potential difference is sufficiently high, the concentration of oxygen on the side facing the solid electrolyte is practically negligible compared to the partial pressure of the gas. According to Fick's first law, the limiting current flowing is a measure of the oxygen partial pressure.

0003The known, based on cubic stabilized ZrO₂ limit current sensors must be operated at temperatures of at least 400 ° C in order to obtain sufficiently large changes in the limit current depending on changes in the oxygen partial pressure. If the use of such limit current sensors is not to be restricted to areas with an already high gas temperature, the limit current sensor must be additionally heated, which delays the readiness for use during commissioning and, in addition, consumes not negligible heat output in view of the high temperatures to be reached. The delayed readiness for use is disadvantageous, for example in the case of λ probes in motor vehicles. The consumption of heating power is particularly disadvantageous in mobile applications. In addition, the high operating temperatures of the limit current sensor are undesirable for a large number of applications, in particular in medical technology applications, in environmental monitoring and in the food industry.

0004It is also known to use cubic stabilized ZrO₂ in potentiometric oxygen partial pressure sensors (H. Dietz, W. Haecker, H. Jahnke in "Advances in Electrochem. & Electrochem. Eng.", Vol. 10, (1977), 46-90). In the case of potentiometric sensors of this type, as is also used in particular with λ probes in motor vehicles, the mode of operation is based on electrochemical reactions (redox reactions) at the phase boundaries between the gas and the solid electrolyte. The resulting redox potential is a measure of the partial pressure of the gas in accordance with Nernst's equation and can be measured as a potential difference based on a reference potential of a reference electrode. A prerequisite for a sufficiently precise measurement is that a phase balance can be established at the phase boundary. When using cubic stabilized ZrO₂, in practice there are sufficiently high balancing rates between gas and electrolyte, in turn, only at temperatures above 500 ° C. In addition to the need to provide a reference electrode, the potentiometric partial pressure sensor has the disadvantage that partial pressure measurements have a logarithmic dependence of the sensor voltage on the partial pressure. For practical evaluation, this method requires a larger partial pressure change with a small change in the residual composition, as is only present in the motor vehicle exhaust gas with a stoichiometric composition. For this reason, measurements at greater distances from the equivalence point are comparatively imprecise and place high demands on temperature constancy and electrode selectivity.

0005From DE-OS 35 43 818 a sensor based on the potentiometric measuring principle for measuring oxygen partial pressures by means of an ion-conducting solid electrolyte is also known. The solid electrolyte consists of tetragonal zirconium dioxide and contains an addition of 2 to 3 mol% of yttrium oxide (Y₂O₃). Such a solid electrolyte allows the equilibrium EMF to be set within seconds or minutes even at a working temperature between 200 and 300 ° C.

0006It is an object of the invention to provide a limit current sensor with which the oxygen gas partial pressure can be measured with comparatively high accuracy even at temperatures below 200 ° C., in particular also in the range of normal room or ambient temperatures.

0007This object is achieved in that the zirconium dioxide of the solid electrolyte element is in its tetragonal, polycrystalline form with a grain size of less than about 4 microns. Surprisingly, it has been shown that tetragonal zirconium dioxide when used as electrolyte material of limit current sensors even at very low temperatures, such as Room temperature, can be brought into equilibrium with oxygen sufficiently quickly, at which the plateau or limit current is essentially independent of the voltage applied to the electrodes of the solid electrolyte and forms a measure of the oxygen partial pressure. The grain size of the tetragonal, polycrystalline zirconium dioxide of less than 4 μm, preferably less than 3 μm, ensures that the tetragonal zirconium dioxide, which is thermodynamically unstable at temperatures below approximately 500 ° C., is stabilized in its monoclinic form even at room temperature, presumably because of the relative Large proportion of the surface energy of the grains compared to their volume with a grain size below 4 µm prevents the phase transition.

0008The cathode side of the electrolyte path of the solid electrolyte element can be provided in a manner known per se with a cover made of gas-tight material which is sealed off from the solid electrolyte element and delimits a cavity therewith. At least one capillary in the cover, but also in the solid electrolyte element, which extends through the cover or the solid electrolyte element into the cavity, limits the gas diffusion into the cavity. Insofar as the capillary is provided in the solid electrolyte element, the solid electrolyte element not only pumps oxygen out of the cavity, but also takes over the diffusion supply of the gas. The capillary can be produced mechanically either during the production of the solid electrolyte element by a thread inserted with the compression process and later pulled out, or also thereafter by means of an ultrasound process or better by a laser beam process. Such processes can quickly produce capillaries with a diameter of 5 to 100 µm. The hole length is typically 1 to 2 mm.

0009Limit current sensors, in which the diffusion barrier lies through a capillary hole in an otherwise gas-impermeable material layer, have comparatively unfavorable dimensions. On the one hand, a long hole with a length which is usually in the mm range is required in the case of a larger oxygen partial pressure (for example in air). On the other hand, a small diameter is required in order to limit the gas diffusion in the gas phase (with the high diffusion coefficients usually present) of the hole. Significantly smaller dimensions and simpler and more reproducible production can be achieved if the diffusion barrier is designed as a solid-state cover made of a material that allows diffusion of the gas to be measured. The solid-state cover is expediently a sintered ceramic oxide material which allows the diffusion of oxygen through the volume, in particular the crystal lattice or the amorphous atomic structure of a glass-like material. The diffusion of oxygen inside a solid is comparatively slow compared to the gas, so that a large cross section and a small thickness of the diffusion barrier are equivalent to the effect of an extremely long and thin hole with gas diffusion. Further advantages of the diffusion barrier designed as a solid-state cover with solid-state diffusion are the simple manufacture and the high degree of conformity of the diffusion from one material to another, since it is a material-specific property. The diffusion flow is essentially determined solely by chemical parameters, ie by the concentration of intrinsic or extrinsic defects. It is particularly important that diffusion barriers made of solid oxide material make it much easier to achieve a defined, highly reproducible, very low diffusion flow than when using gas-impermeable covers with a capillary hole. Although the configuration of the diffusion barrier as a solid body cover is particularly suitable for limit current sensors with a solid electrolyte element made of tetragonal polycrystalline zirconium dioxide of the type explained above, it can also be used for limit current sensors with a solid electrolyte element made of another material. The claims directed to the diffusion barrier made of a solid oxide material are therefore of independent importance. For example, the solid-state diffusion barrier can also be used for other gases that can be detected with a solid electrolyte.

0010The solid-state cover can be arranged similarly to the diffusion barrier formed by a capillary hole in a gas-impermeable cover to form a cavity at a distance from the solid electrolyte element, whereby here too it is sealed at the edge of the solid electrolyte element by a suitable sealing material such as glass or epoxy resin etc. However, there are significant advantages if the solid body cover and the solid electrolyte element are formed as layers lying flat on one another. This not only has the advantage of greater mechanical stability and simpler manufacturing processes, but also enables considerable miniaturization, since the layers can be applied to a suitable substrate material, such as porous ceramic or the like, using thin-film or film technology. The diffusion barrier can be evaporated directly onto the solid electrolyte layer, sputtered on or applied in an otherwise conventional layer production method. The solid electrolyte manufactured using thin-film technology also has a very short response time compared to conventional, thick solid electrolyte elements. In particular at low temperatures, solid electrolyte elements produced in thin-film technology show only a comparatively small influence of ohmic polarization of the solid electrolyte and often a widening of the plateau area. The solid electrolyte element of the limit current sensor for the detection of high oxygen partial pressures at low temperatures is expediently also produced using thin-film technology if the diffusion barrier delimits a cavity toward the solid electrolyte element. The thickness of the film is advantageously less than 0.5 mm, in particular less than 0.2 mm.

0011The oxide solid-state cover serving as a diffusion barrier should let the oxygen through uncharged. This can be done, for example, by oxides with mixed conductivity, such as binary or multinary oxides made of, for example, NiO, Cu₂O, CuGeO₃ or Y Ba₂Cu₃O_{6 + x}. can be achieved. However, ionically conductive solids are also suitable, such as the tetragonal polycrystalline zirconium oxide also used for the solid electrolyte element, if the electrons are guided through an external electrical circuit instead of through the interior of the solid, so that together there is a neutral oxygen flow. The advantage of the diffusion barrier consisting of the material of the solid electrolyte element is that it has the same coefficient of thermal expansion, which makes it easier to seal against the solid electrolyte element.

0012In the case of mainly ion-conducting oxide materials, the chemical diffusion coefficient determining the oxygen diffusion is determined by the diffusion coefficient of the electronic minority charge carriers. This blocking and reduction of the oxygen diffusion to a very low value can be avoided if the solid electrolyte cover carries electrodes which are permeable to the gas to be measured on both sides of its diffusion path and which are conductively connected to one another via a short-circuit connection or an applied voltage or a forced current. The short-circuit connection enables the compensation of the electronic charge carriers, so that the chemical diffusion coefficient is determined by the diffusion of the ions of the gas to be measured, here the oxygen ions. The applied voltage or the forced current allows the diffusion coefficient to be checked or controlled. As the ion-conducting material of the diffusion barrier, it is therefore expedient to use a solid material which specifically conducts the gas ions of the gas to be measured, for example ZrO₂ in the case of oxygen.

0013The electrodes of the electrolyte line can be applied directly to the opposite surfaces of the solid electrolyte element. Porous platinum electrodes which allow the passage of gas are suitable, for example vapor-deposited, sputtered on or applied in the form of conductive lacquer. Insofar as the diffusion barrier lies flat against the solid electrolyte element, it can be applied directly to the electrode of the electrode path, which is now in between. Insofar as the diffusion barrier consists predominantly of electronically conductive oxide solid material, it can also be applied directly to the oxide solid electrolyte element, the electrodes of the electrolyte path enclosing the layer structure consisting of diffusion barrier and solid electrolyte element between them. A third electrode is required between the diffusion barrier and the solid electrolyte element if the diffusion barrier is made of an ionically conductive material, the two electrodes enclosing the diffusion barrier between them in turn being connected to one another by a short-circuit connection or a voltage or current source.

0014Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. Here shows:<ul id="ul0001" list-style="none"><li>1 shows a schematic sectional view of a limit current sensor with a capillary diffusion barrier;</li><li>FIG. 2 shows a variant of the limit current sensor according to FIG. 1;</li><li>3 shows a schematic sectional view through a limit current sensor with a solid-state diffusion barrier;</li><li>4 shows a schematic sectional view of a limit current sensor with an ion-conducting solid-state diffusion barrier;</li><li>5 shows a schematic sectional view through a limit current sensor with an electrically neutral conductive diffusion barrier;</li><li>FIG. 6 shows a schematic sectional view through a variant of the limit current sensor from FIG. 5;</li><li>7a and 7b are diagrams showing the dependence of the limit current i on the electrolyte voltage U and the oxygen partial pressure p at a sensor temperature of 600 ° C. and</li><li>8a and 8b corresponding to FIGS. 7a and 7b diagrams for a temperature of the limit current sensor of 250 ° C.</li></ul>

0015The limit current sensor shown schematically in Fig. 1 comprises a layered solid electrolyte element 1 made of tetragonal, polycrystalline zirconium dioxide (ZrO₂), which carries electrodes 3, 5 made of porous platinum on both sides. The electrode 3 is covered with a cavity 7 from a gas-tight cover 9, made of a crystalline or vitreous solid, but preferably also made of ZrO₂, and by an edge seal 11, for example made of glass or epoxy resin, sealed to the solid electrolyte element 1. The cover 9 contains a capillary 13 with a diameter between 5 to 100 microns and a length of about 1 to 2 mm. While the electrode 5 is essentially freely exposed to the gas whose oxygen partial pressure is to be determined, the capillary 13 forms a diffusion barrier which limits the inflow of the gas to the cavity 7 and thus to the electrode 3. A DC voltage source 15 is connected to the porous electrodes 3, 5 via electronic feed lines 17, 19. The DC voltage source 15 is connected so that the electrode 3 facing the cavity 7 and thus the diffusion barrier forms the cathode and the opposite electrode 5 forms the anode.

0016The solid electrolyte element 1 consisting of tetragonal, polycrystalline ZrO₂ is an oxygen ion conductor and "pumps" oxygen out of the cavity 7 due to the voltage U. If the electrode voltage U is sufficiently high, the pump power exceeds the diffusion flow, with the result that the oxygen partial pressure in the cavity 7 becomes practically zero. The current i flowing here reaches a plateau or Limit value which is proportional to the oxygen partial pressure according to the following formula: i = - 4F<img file="EP0408039A2_D0001.tif" /> ln (1-P /<o ostyle="single">P</o>)

0017Here means: D the diffusion coefficient of oxygen in the gas, S the cross section of the capillary 13, L the length of the capillary 13, F is the Faraday constant, R the general gas constant, T the absolute temperature,<o ostyle="single">P</o> the total pressure of the gas. P is the partial pressure of oxygen in the gas. The limit current is measured by means of a current measuring device 21.

0018The above equation takes into account that the free path length of the gas molecules increases as the oxygen partial pressure decreases in the cavity 7.

00197a and 8a show measured values ​​of the current i as a function of the electrolyte voltage U for different values ​​of the oxygen content in the measuring gas consisting of CO₂, CO, H₂, Ar and O₂ at temperatures of 600 ° C and 250 ° C. 7b and 8b show the relationship between the measured plateau or Limit current values ​​and the limit current values ​​indicated by crosses and calculated according to the above equation for different values ​​of the oxygen partial pressure p. 7a and 8a show, the current i increases again with increasing voltage U, which can be attributed to increasing electronic conductivity and the decomposition of the electrolyte. The increase in the current i at a very low voltage U is due to the fact that the oxygen partial pressure in the cavity 7 is not negligible compared to the oxygen partial pressure of the gas, as is necessary for the limit current view. In addition, the ionic resistance of the electrolyte determines the pumping rate in this area, so that the plateau current is not reached due to ohmic polarization losses.

0020As shown in FIGS. 8a and 8b, the plateau or limit current is also set for lower temperatures than was previously customary for limit current sensors. For low oxygen partial pressures, a thin, conventionally manufactured tablet with a thickness of 0.1 to 1 mm is sufficient to detect the oxygen partial pressure at room temperature or slightly elevated temperature. At higher oxygen partial pressures, the range of the ohmic increase in the iU diagram overlaps the limit current range, so that thick-film or thin-film films have to be used.

0021Tetragonal, polycrystalline ZrO₂ can be produced based on the process described in DE-OS 35 43 818 with a grain size of less than 4 microns, preferably less than 2 to 3 microns. Due to the comparatively small grain size, the tetragonal ZrO₂, which is inherently unstable below 500 ° C., is temperature-stabilized.

0022Variants of the limit current sensor are explained below. Components having the same effect are provided with the reference numbers of FIG. 1 and with a letter to distinguish them. To explain the structure and the mode of operation, reference is made to the description of FIG. 1.

0023Fig. 2 shows a variant of the oxygen limit current sensor from Fig. 1, which differs from this essentially only in that the capillary 13a serving as a diffusion barrier is not provided in the gas-tight cover 9a, but directly in the solid electrolyte element 1a. The solid electrolyte element 1a thus has both the function of the diffusion barrier and that of a "pump element". Similar to the limit current sensor of FIG. 1 The capillary 13a can be inserted by letting a thread in and then pulling it out later during the compression process of the solid electrolyte or mechanically incorporated by means of ultrasound methods or preferably by laser beam methods. The capillary 13a can be produced both during the production of the solid electrolyte element 1a and after its production. The gas-tight cover 9a is in turn sealed to the solid electrolyte element 1a to form a cavity 7a and expediently again consists of ZrO₂. The solid electrolyte element 1a consists of tetragonal, polycrystalline ZrO₂, as was explained with reference to FIG. 1.

0024Fig. 3 shows an oxygen limit current sensor, which consists of tetragonal, polycrystalline ZrO₂ solid electrolyte element 1b sealed to form a cavity 7b covered by a cover 9b, which for forming a diffusion barrier from a gas diffusion of oxygen solid body made of sintered ceramic-like oxide material. The solid electrolyte element 1b in turn consists of tetragonal, polycrystalline ZrO₂ of the type explained above and carries porous electrodes 3b, 5b connected to the DC voltage source 15b on both sides. The cover 9b, which is connected to the solid electrolyte element 1b in a gas-tight manner via seals 11b, consists of binary or multinary oxides with neutral conductivity. Oxygen is transported through the crystalline or glass-shaped interior of the diffusion barrier without having to work a capillary hole. Since the cover 9b can also be comparatively thin, the limit current sensor has smaller dimensions than the sensors explained above. The limit current i is approximately proportional to the oxygen partial pressure according to Fick's 1st law: i ∼<img file="EP0408039A2_D0002.tif" />p.

0025Here means i in turn the plateau or limit current,<img file="EP0408039A2_D0003.tif" /> the chemical or effective diffusion coefficients, p the oxygen partial pressure.

0026NiO, Cu₂O, CuGeO₃ or Y Ba₂Cu₃O is suitable as material for the cover 9b_{6 + x}.

0027The cover 9b of the limit current sensor according to FIG. 3 consists of oxides which, by means of mixed conductivity, ensure local charge neutrality. The limit current sensor of FIG. 4 differs from the sensor of FIG. 3 in that its cover 9c, which forms the diffusion barrier, consists of an ion-conducting solid oxide material, in particular tetragonal, polycrystalline ZrO₂. Since the solid electrolyte element 1c and the cover 9c are made of the same materials, the temperature resistance of the seal 11c is excellent due to the same temperature expansion coefficient. The cover 9c is covered on both sides with porous electrodes 23, 25 which are permeable to the gas and which are electrically conductively connected to one another by a short-circuit connection 27 or a voltage or current source indicated at 28. This achieves a balance of the electronic minority charge carriers of the mainly ionically conductive solid material of the cover 9c, so that the chemical diffusion coefficient is not determined by the diffusion coefficient of the electronic minority charge carriers. The short-circuit connection or the voltage or current source ensures that the balance of the electronic charge carriers is not blocked and the diffusion rate is determined by the oxygen ions and, if appropriate, the external voltage or the current.

00285 shows an oxygen limit current sensor in which the cover 9d serving as a diffusion barrier is applied directly to the solid electrolyte element 1d, for example vapor-deposited or sputtered on. At least the cover 9d, but preferably also the solid electrolyte element 1d, is designed as a thin layer, so that the sensor can be miniaturized without problems. A gas-tight seal 11d made of glass or epoxy seals the contact of the cover, the solid electrolyte and the gas in order to avoid a transition of the oxygen at this point directly from the glass into the solid-state electrolyte without diffusion through the entire thickness of the cover. The oxide material of the cover 9d that permits gas diffusion is electrically neutral. The electrodes 3d, 5d, which in turn consist of gas-permeable, porous platinum, enclose the layer structure of the cover 9d and the solid electrolyte element 1d between them and are applied directly to the layer structure. The electrode 3d applied to the cover 9d forms the cathode. Alternatively, however, the electrode 3d can also be arranged between the cover 9d and the solid electrolyte element 1d, the electrode 3d being applied first and then the cover 9d lying directly on top of it during manufacture. In both variants, the cover 9d consists of a solid oxide material with local charge neutrality. The solid electrolyte element 1d again consists of tetragonal, polycrystalline zirconium dioxide.

0029Fig. 6 shows a variant of the limit current sensor from Fig. 5, in which the in turn flat on the solid electrolyte element 1e without a cavity as a diffusion barrier cover 9e made of a mainly ion-conducting material, in particular corresponding to the solid electrolyte element 1e made of tetragonal, polycrystalline ZrO₂ solid material consists. The electrodes 3e, 5e of the electrolyte line consist of porous platinum and are applied to both sides of the layered solid electrolyte element 1e, the cathode-side electrode 3e being arranged between the cover 9e and the solid electrolyte element 1e. Opposite the electrode 3e, the cover 9e carries a further electrode 29, which corresponds to the limit current sensor of FIG. 4th is electrically conductively connected to the electrode 3e by means of a short-circuit connection 31 or a voltage or current source indicated at 32. The short-circuit connection 31 or the voltage or current source 32 ensures the compensation of electronic minority charge carriers of the mainly ionically conductive solid material of the cover 9e.

0030Although the limit current sensors explained above are functional at very low temperatures due to the use of tetragonal, polycrystalline ZrO₂ as solid electrolyte material, additional heating can be provided in individual cases. The heater can also be arranged on the sensor element or can be formed by utilizing existing electrode leads.

0031In the exemplary embodiments in FIGS. 1 and 3 to 6, the solid electrolyte elements are expediently designed as thin layers or films with a thickness of, for example, 0.1 mm. This allows the measurement of high oxygen partial pressures, for example in air at room temperature. The thin electrolyte layer has only a low resistance, as a result of which the rectilinear region of the current rise in the low voltage region of FIG. 7a and 8a becomes steeper and the plateau area can widen.