Abstract:
Oxygen sensitive resistance materials for use with oxygen sensors, in particular λ probes are described. These materials are based on the fact that with complex metal oxides it is possible—by adding suitable doping substances—to achieve a negligible temperature dependence of the electric resistance of these materials for different oxygen partial pressures and to preset it to a desired partial pressure value.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part application, filed under 35 U.S.C. §120, of PCT Application Ser. No. PCT/DE98/02937, filed Oct. 5, 1998 (designating the United States), which claims priority on German Patent Application DE 197 44 316.8-52, filed Oct. 8, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention concerns a temperature independent oxygen sensitive resistance material on a titanate basis. 
     2. Description of the Related Art 
     Ever stricter exhaust gas limit values, in combination with the pressure to reduce fuel consumption, force automotive manufacturers to develop new concepts for combustion engines. It becomes clear that the above-mentioned requirements are best brought together when operating with excess air (air ratio λ&gt;1). Such modern “lean concepts” require exact information on the oxygen content found in the exhaust gas. The principle of the standard potentiometric λ probe can be designed with difficulty and some great effort only for high oxygen concentrations as they occur in such lean exhaust gases. 
     In order to be able to measure the oxygen content of the exhaust gas even within the lean area, it was proposed, for example, in EP 0 191 627, in DE 38 41 611, as well as in [1] to set up amperometric probes in accordance with the current limit principle (“Limit current probe”) from a material containing oxygen ions. 
     Such a limit current sensor is, for instance, also contained in Chemical Abstract 191612 for JP 02269948, such that a fixed electrolytic body fitted with two electrodes located opposite each other is introduced, where one of the electrodes consists of a Perovskite type oxide with the general formula L (1−x) A (x) Co (1−y) M (y) O (3−delta) , with A being Sr, Ba, Ca and L being La, Ce, Pr, Nd, but where no generic type (Ti 1−z Fe z ) complex can arise, as for M only Fe, but not Ti and definitely no mixture of the two can be taken, which, however, according to the state of the art described below, would be fundamental for achieving a temperature independence from the working point of the oxygen partial pressure. 
     In addition, the operating mode of this sensor differs completely from resistive sensors and is based on an ion diffusion capability of an electrically insulating, but porous and for oxygen ions diffusion-capable layer, which is limited by a further gas-diffusion-controlling layer such that for an appropriate pump voltage a depletion in oxygen ions occurs, and the current flow to be measured now becomes dependent on the oxygen partial pressure. Any possibly occurring change in the ohmic resistance of a material of one of the electrodes will thus not be considered or utilized, and is rather regarded as an interfering factor. 
     However, according to DE 2334044 as well as pursuant to [2] there is also the option to utilize the oxygen partial pressure dependence of the electric conductivity of a metal oxidic material as a sensor effect, and to produce a sensor from this material, on the basis of whose electric resistance R it is possible to draw conclusions as to the oxygen partial pressure PO 2  of the exhaust gas, and, on that basis again, as to the oxygen content in the exhaust gas. 
     Doped titanium oxide (TiO 2 ) and strontium titanate (SrTiO 3 ) were investigated with particular thoroughness (DE 37 23 051. 
     EP 0 365 567, [2]), as—due to their chemical stability—such titanium oxides are capable of withstanding the extreme operating conditions within the exhaust gas train of a combustion engine. 
     However, sensors made up of these compounds—as of most other metal oxides—feature a very significant temperature dependence of the electrical resistance, which requires an extensive heating control system, in combination with major construction measures that will reduce the influence and effects of sudden changes in temperature. 
     Therefore, it was considered in DE 42 02 146, DE 42 44 723, DE 43 25 183 and EP 0 553 452 to use doped or undoped cuprates in future such as e.g. La 2 CuO 4+y , as their electrical conductivity—specifically in the area of high oxygen content values, that is for μ&gt;1,—is temperature independent. For applications within the exhaust gas train, however, cuprates are not suitable, as they are chemically not all that stable and decompose at high temperatures and/or low oxygen partial pressures, such as e.g. during short term operation with a “rich” mixture (λ&lt;1). 
     Lanthanferrites doped with alkaline earth, known from DE 44 18 054, feature a significantly higher chemical stability than cuprates. Their electrical conductivity also features a lower temperature dependence within the lean exhaust gas range (λ&gt;1) when compared to SrTiO 3 . However, sensors made from these materials show a greater temperature dependence of the electrical resistance value than sensors on a cuprate basis. 
     In EP 0 062 994 or in U.S. Pat. No. 4,454,494 with the same priority, Williams et al. proposed to replace the titanium (Ti) in SrTiO 3  in part by iron (Fe), and found out that sensors made from the compound SrTi 0.7 Fe 0.3 O 3−δ in lean atmospheres above 500° C.-600° C. have almost no temperature dependence of the electrical resistance but show a dependency on partial oxygen pressure according to R˜pO 2   {fraction (−1/5)} . Own measurements confirmed this but also showed that the temperature independence of the electric resistance will exist only at a partial oxygen pressure around 10 −2  bar (λ≈1.055). 
     SUMMARY OF THE INVENTION 
     The present invention states oxygen sensitive resistance materials based on complex metal oxides where the disappearing temperature dependence of the electrical resistance of the sensor can be adjusted by adding suitable doping substances with regard to the oxygen partial pressure. 
     The material composition according to the invention has the advantage that the range for a complete temperature independence, that is, TKR=0, where TKR states the temperature coefficient of the electrical resistance of the sensor at a constant oxygen partial pressure of              TKR   =       1   R     ×       Δ                 R       Δ                 T                     pO   2       =     const   .                            
     can be specifically shifted—by adding suitable doping materials—toward high as well as low oxygen partial pressure values. Due to this material composition it is possible to design the sensor such that the sensor signal of the sensor made therefrom will be independent of the actual temperature for specific engine concepts, which are characterized by a specific oxygen content in the exhaust gas. In respect of any such sensor, there is no need for a complex and costly heating control system, which produces a major cost advantage when compared to known principles. In addition, the sensor can be designed such, by adding further doping materials in accordance with the invention, that the temperature coefficient of its electrical resistance will still remain negligble even in the event of greater deviations from the above-mentioned working point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the resistance R of a sensor, built up from a material of material category  1   a  (Sr 1−a M a Ti 1−z Fe z O 3−δ ), here as an example with z=0.3, M=Lanthan (La), and a=0.05) applied in a double logarithm against the oxygen partial pressure PO 2  of the exhaust gas. The parameter is the temperature T of the sensor. 
     FIG. 2 shows the resistance R of a sensor, built up from a material of material category  2   a  (Sr(Ti 1−z Fe z ) 1−c M″ c O 3−δ ), here as an example with z=0.3, M″=Gallium (Ga) and c=0.05) applied in a double logarithm against the oxygen partial pressure PO 2  of the exhaust gas. The parameter is the temperature T of the sensor. 
     FIG. 3 shows the comparison of the temperature coefficient of the resistance TKR for three sensors, built up from materials of the material classes  1   a ,  2   a , and  3 , where material class  3  represents a mixture made up of material classes  1  and  2 . By way of example, a mixture ratio of 1:1 from material classes  1   a  and  2   a  was selected here, with z=0.3, M=Lanthan (La), a=0.05, M″=Gallium (Ga), and c=0.05. 
     FIG. 4 shows an embodiment in thick film technology. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Using a material composition in accordance with the invention, the temperature independence range of the electric sensor resistance can be shifted into the range of λ&gt;2, that is PO 2 &gt;0.1 bar, which is the relevant range for Diesel engines and “lean Otto engines”. This can be seen quite clearly from FIG.  1 . There, the resistance of a sensor, built up from a material composition in accordance with the invention, is plotted against the oxygen partial pressure of the exhaust gas. Here, in the original compound SrTi 1−z Fe z O 3−δ one part of the strontium, which occurs as a bivalent, was replaced by a trivalent ion of class M, which embeds itself in a strontium position. This material composition, in the following designated as material class  1   a , thus is: p 1  Sr 1−a M a Ti 1−z Fe z O 3−δ . 
     By way of example, M=Lanthan (La), a=0.05 and in accordance with EP 0 062 994 z=0.3 were selected in FIG.  1 . The same effect will also be achieved by replacing strontium with quadrivalent ions M, which embed themselves in a strontium position. Also, replacing titanium and iron by pentavalent or hexavalent ions M′ in accordance with Sr(Ti 1−z Fe z ) 1−b M′ b O 3−δ (material class  1   b ) will lead to the same result. In FIG. 1, it can be clearly seen how the temperature independence range of the sensor resistance value has shifted to higher oxygen partial pressures in comparison with EP 0 062 994; here, for example, to pO 2 ≈0.1 bar, that is λ≈2). 
     The complete or partial replacement of the bivalent strontium by means of another bivalent element N, e.g. N 1−a M a Ti 1−z Fe z O 3−δ or (Sr 1−n N n ) 1−a M a Ti 1−z Fe z O 3−δ , or N(Ti 1−z Fe z ) 1−b M′ b O 3−δ or (Sr 1−n N n ) (Ti 1−z Fe z ) 1−b M′ b O 3−δ , is to be regarded as being included in material class  1 . 
     Typical examples for N are Barium (Ba) or Calcium (Ca). Here, n may assume values from zero (strontium is not replaced) to one (strontium is fully replaced), without there being a fundamental change in the sensor characteristics. δ characterizes the oxygen deficit which occurs, depending on the composition, due to the electron neutrality condition. 
     Also included in material class  1  are materials in accordance with the above-mentioned composition, in which the quadrivalent titanium is replaced fully or partially by one or several quadrivalent elements. Here, too, sensor characteristics change insignificantly only. Typical examples for such quadrivalent elements are zirconium (Zr) and tin (Sn). The titanium may also be fully or partially replaced with tin (Sn), silicon (Si), germanium (Ge), cerium (Ce), and hafnium (Hf). 
     FIG. 2 shows how the temperature independent range of the sensor resistance shifts to lower oxygen partial pressures and thus to a lower exhaust gas oxygen content due to the partial replacement of titanium or iron by a trivalent or bivalent ion M″ in accordance with Sr(Ti 1−z Fe z ) 1−c M″ c O 3−δ (material class  2   a ). By way of example, this is shown here by z=0.3, M″=Gallium (Ga) and c=0.05. The same effect is achieved by replacing the strontium with a monovalent ion M′″. The composition will then be Sr 1−d M′″ d Ti 1−z Fe z O 3−δ (material class  2   b ) .Using material classes  1  and  2 , a material system will thus be available which, depending on its composition, will provide for the manufacture of a resistive oxygen sensor, whose sensor resistance features a disappearing temperature dependence range that can be shifted towards any desired oxygen partial pressures. The same applies to replacing strontium by a bivalent ion, or replacing the titanium by a quadrivalent ion. 
     In FIG. 2 it is to be noted that in the range, within which the sensor resistance still features a temperature dependence, its TKR is &lt;0, whilst the sensor characteristic from FIG. 1 in the range that is not temperature independent has a TKR&gt;0. From a combination of two sensors built up from material classes  1  and  2 , which are electrically connected in a suitable fashion, it is thus possible to produce a sensor which, across the entire exhaust gas range occurring for lean combustion engines, has a still lower or—if a suitable composition is selected—disappearing temperature dependence. But such a combination may also be any given combination of material classes  1  and  2  to a material class  3 , from which the sensor will then be built up. Such an example is shown in FIG.  3 . Due to a particularly suitable combination of material classes  1   a  and  2   a , and here in the ratio 1:1, the temperature coefficient of the sensor resistance—across the entire temperature range—will be of a smaller amount than for any material class  1   a  or  2   a  on an individual basis. A major variation of the temperature coefficient of the sensor resistance and its temperature independent oxygen partial pressure range can be achieved by a suitable combination of the parameters a and/or b, c and/or d and the mixture ratio. 
     FIG. 4 shows a possible very simple embodiment  10  in thick film technology. On the top surface  14  of a substrate  12  there are four electrical connection tracks  20 ,  22 ,  24 ,  26 . At one end  30 , the connection tracks are covered, as drawn, by a gas sensitive film  32  consisting of material classes  1  or  2  or  3 . At the opposite end, terminal  34  is provided with contact surfaces for attaching electric supply lines. In this way, the film resistance can be measured in four-wire technology. Then the contact resistance between electrode and film material will no longer play any part. On the underside  16  of substrate  12  a heater track  40  is applied, which heats the gas sensitive film to the temperature range shown in FIGS. 1 to  3 . A precise temperature control system is not required in the case of the material compositions in accordance with the invention. An associated sensor housing is not shown here. 
     Below, in an example, the production of a powder in accordance with the invention for material class  1   a ) is described. SrCO 3 , M 2 O 3 , TiO 2 , and Fe 2 O 3  will be weighed in such that the desired stoichiometry ratio is achieved. For z=0.3, a=0.05, and M=La, a mass ratio of SrCO 3 , La 2 O 3 , TiO2, and Fe 2 O 3 , as calculated from the molar masses, must be weighed in such that the atomic ratio Sr:La:Ti:Fe is 19:1:14:6. As La 2 O 3  is hygroscopic, it is first annealed at approx. 850° C. and then weighed in when hot. A typical preparation of approx. 50 g raw powder will then be filled into a grinding cup together with a grinding medium, for which e.g. a solvent such as e.g. cyclohexane or isopropanol is used; grinding balls, made of agate for instance, diameter 10 mm for a quantity of 50 pcs. or a lower ball diameter for a correspondingly greater quantity of grinding balls, are added; the entire preparation will then be mixed in a planetary ball mill for a period of one to four hours. The grinding material mixed in this way will then be dried, separated from the grinding balls, filled into a crucible, and fired for 15 hours in a furnace in air atmosphere at 1200° C. The cooled down powder will then have the desired material composition which is shown e.g. by x-ray diffractometer analysis. A powder fired in this way must still be crushed by means of a further grinding stage, in order to achieve a powder grain size distribution suitable for the further sensor production process. A typical grinding process is executed as described above, but to this end 7-10 balls with 20 mm diameter each are used. Also, powder crushing is effected in an attritor mill or an annular clearance ball mill. The dried powder, separated from the grinding balls, will then be used for sensor production. 
     The powders of material classes  1   b ,  2   a ,  2   b , and  3  are also produced in analogy to the process described. Depending on the composition, it will then be possible to replace M 2 O 3  with the corresponding oxides, carbonates, or oxycarbonates as base material. These will then be weighed in according to the then valid stoichiometry. 
     From the original materials made in accordance with example 1, sensors are produced as follows. The powders will be brought into a suitable form; if necessary, a suitable binding agent can be used for this purpose, pressed, and sintered in air at temperatures between 1300° C. and 1400° C. The typical dimensions of a sinter body are 25 mm×3 mm×0.2 mm. This sensor body will then be contacted with platin wires, which are fixed with a platin firing paste. The paste can be fired at e.g. 1050° C. Two, three, or four such platin electrodes will be applied. Correspondingly, the resistance measurement will then be effected by means of two wire, three wire or four wire measurement. A single heater, consisting of a heater wire and an electrical insulation, which is necessary to separate sensor and heater from one another, still needs to be fitted, in order to compensate an exhaust gas temperature which is too low, and to bring the sensor up to operating temperature as early as possible. Finally, the sensor is introduced into a suitable protective housing. 
     Alternatively, a sensor in accordance with the invention can be built up in thick film technology. The powders made from the original materials produced in accordance with the above-mentioned process are mixed with suitable media which render the original material capable of screen printing. On a substrate made of e.g. Al 2 O 3  or ZrO 2 , appropriate connection tracks, made of gold or platin, are printed in screen printing technology, in order to be able to measure the sensor resistance in a two wire, three wire, or four wire arrangement. These tracks are typically burnt in in an air atmosphere. Then the sensor film will be printed and also burnt in. On the back of the sensor, a further heating film will be applied which features the above-described functions. Supply lines are connected to the electrodes of the measurement tracks and heater tracks. Setup takes place in accordance with FIG. 4. A suitable housing provided with electrical feedthroughs ensures mechanical stability and protects the sensor. 
     The above-described oxygen sensitive resistance materials are suitable for a multitude of applications. Thus there is a provision for using oxygen sensors to determine the oxygen content in fuel/air mixtures, in the exhaust gas of combustion engines, in plants for the removal of nitrogen or desulphurization plants, in sinter furnaces, in bioreactors, and in facilities, which due to combustion—especially of carbon-, hydrocarbon- or hydrogen-rich fuels—produce thermal energy as usable energy. Here, a respective adaptation of the resistance material to the respective existing oxygen partial pressure is provided, in order to ensure to a large extent that temperature independence is achieved in this working point. 
     LITERATURE 
     [1] Kleitz M., Siebert E., Fabry P., Fouletier J.: Solid-State Electrochemical Sensors. 
     In: Sensors. A comprehensive Survey. Chemical and Biochemical Sensors Part I. Göpel W. et al. (Publ.), VCH-Verlag, Weinheim, 1991, Page 341-428. 
     [2] Schönauer U.: “Dickschicht Sauerstoffsensoren auf der Basis keramischer Halbleiter.” (Thick film oxygen sensors based on ceramic semiconductors). Technisches Messen 56[6] 260-263, 1989.