the zirconium dioxide composition may additionally comprise inorganic dopants . possible inorganic dopants are cao , mgo , ceo and y 2 o 3 . the zirconium dioxide composition is preferably doped with y 2 o 3 . in this manner , a large number of vacant sites is generated for o 2 - ions , which make the ceramic ionically conductive . in the range of from 3 . 5 to 14 . 0 % by weight of y 2 o 3 , preferably of from 5 to 10 % by weight , based on the total amount of zirconium dioxide composition , the ionic conductivity is particularly suitable for the intended use of the sensor downstream of the catalyst . the correct choice of the y content also improves the stability of the sensor against thermal shock . oxide mixtures comprising 5 % by weight of y 2 o 3 produce purely tetragonal grains during sintering . mixtures comprising 10 % by weight of y 2 o 3 produce a mixture of tetragonal and cubic grains . in both cases , a so - called internal work hardening inhibits or entirely prevents unwanted spreading of cracks in the ceramic . the use of a zirconium dioxide comprising y 2 o 3 is known , inter alia , from german patent 35 37 709 . according to the invention , the zirconium dioxide layers are arranged in such a way that , on a first layer , an electrode made of platinum is arranged which is applied in the form of a paste comprising the metal as a finely particulate powder together with organic binders and , if required , solvent . on the first layer with the electrode made of platinum a second layer made of zirconium dioxide is arranged which has a central cutout , which in turn has a connection to an external edge of the layer . the termination is formed by a third layer made of zirconium - dioxide , which , on its surface which faces away from the second layer having the cutout , carries two electrodes which are preferably arranged parallel and which are made of platinum and of a platinum alloy . the electrodes of the third layer are likewise preferably applied in the form of pastes which comprise platinum , or the alloy , as a finely particulate powder , together with organic binders and additionally , if required , solvent . the electrodes may , however , alternatively be applied by other conventional methods , for example by sputtering , by electrodeposition or according to the chemical vapor deposition method , in which previously formed volatile precursors are carried to the substrate surface and are decomposed there . the layer thicknesses of the individual zirconium dioxide layers can be different and are in the range of from 0 . 2 to 2 . 0 mm , preferably of from 0 . 5 to 1 . 1 mm , particularly preferably of from 0 . 5 to 0 . 9 mm . on that side of the three - layer combination of zirconium dioxide layers which does not carry any electrodes , the electric heating element comprising two aluminum layers and an electric heating resistor is arranged according to the invention . the heating element permits a reliable determination of the combustion exhaust gases by the zirconium dioxide sensor element even when these are still cold or have not yet reached a particular operating temperature . the heating element has a layered structure , the electric heating resistor having a preferably meander - like course in the interior of the electrically insulating ceramic made of al 2 o 3 and being constructed from a suitable refractory metal of low conductivity ( e . g . tungsten ). two contacts pointing outwards allow the electric heating resistor to be connected to a power source . expediently , the heating element is connected , via a temperature - dependent resistor , in such a way that at elevated temperatures of the combustion exhaust gases to be measured the heating output of the heating element is reduced . the heating element can be connected mechanically ( e . g . by a clamping mechanism or a pressure spring ) to the actual sensor composed of zirconium dioxide layers and electrodes . for use , the sensor is usually installed in a suitable housing in such a manner that the layer planes are parallel to the axis of the housing . the housing can expediently be constructed from corrosion - resistant metal , for example from high - grade steel , the interior of the housing being subdivided into two chambers in a gas - tight manner by a ceramic element , one of which chambers is in direct contact with the combustion exhaust gases , the other chamber being insulated against the combustion exhaust gases . the zirconium dioxide sensor element according to the invention is arranged in such a way that the free electrodes are located in the chamber containing the combustion exhaust gases , while the voltage pick - off takes place in the insulated chamber . the voltage pick - off itself should be carried out in an isolated circuit in order to minimize the number of possible error sources during the measurement . the housing is provided on its outside with connecting wires and plugs to make it suitable for fitting in internal combustion engines of motor vehicles or in combustion plants . the geometric dimensions of the areal sensor can be chosen at will . for example , it may be rectangular , square or round . because of its suitability for fitting , an approximately elongated , rectangular shape is preferred . the overall thickness of the sensor is preferably approximately from 2 to 6 mm . the external dimensions are preferably 10 × 50 mm . it has been found , surprisingly , that the construction according to the invention of the sensor , in spite of its simplicity , enables quantitative detection of the most important individual components of a mixture of combustion exhaust gases . according to the functional principle of the determination , the partial pressure of an individual component is first registered by means of the voltage difference between the reference electrode inside the sensor and a first measuring electrode . this signal is related to the voltage difference between the reference electrode and the second measuring electrode . owing to the different chemical compositions of the two measuring electrodes , the individual signals allow inferences regarding different subcombinations of individual components of the overall mixture and , according to the abstraction principle , produce values for the individual components themselves . if yet more individual components were required to be determined , it may be expedient to provide yet a third measuring electrode having a different chemical composition on the sensor according to the invention . the invention further relates to a process for fabricating the planar sensor described above , which comprises zirconium dioxide and aluminum oxide and is suitable for measuring the proportions of oxygen and combustible gases in the exhaust gases of internal combustion engines or furnaces . the process comprises casting a liquid ceramic tape - casting composition comprising organic additives , especially a volatile organic solvent , finely dispersed zirconium dioxide and optionally inorganic dopants , onto a flat substrate and generating a thin film in the process , drying the film and detaching it from the substrate , punching a plurality of equal - sized cards from the dried film , which approximately correspond to the dimensions of the eventual sensor applying onto the surface of a first card , by printing a platinum metal paste thereon , an electrode which extends at least as far as the central region of the card which is connected to the edge of the card via a printed conductor , applying onto this first surface at least one second card which in its central region has a cutout , applying onto this second card at least one third card which carries two platinum and platinum alloy electrodes extending at least as far as the central region of the card , which electrodes are applied by corresponding pastes being printed on and which are connected to the edge of the card via printed conductors , in such a manner that the surface having the platinum and platinum alloy electrodes faces away from the second card having the cutout in the central region , and firing this card structure oxidatively at from 1400 ° to 1600 ° c . the three cards sintering together to form a zirconium dioxide sensor having a planar structure . the liquid ceramic tape - casting composition comprises , as organic additives , especially binders , dispersants , a volatile solvent ( e . g . trichloroethylene / alcohol ) and plasticizers . the substrate used for casting can be a steel plate , a mobile steel strip or a smooth plastic film , e . g . a polyester film . the thickness of the resulting film is approximately from 0 . 2 to 2 mm , preferably from 0 . 3 to 0 . 9 mm . a constant thickness of the thin film can be achieved by using a casting shoe . the film can also be dried continuously . the dimensions of the cards punched out shrink during ceramic firing , depending on the volume proportion of organic components , by from 10 to 30 %, especially from 22 to 25 %. the invention is explained below in more detail by way of example with reference to the accompanying figures , without however being restricted to the concrete embodiment shown . fig1 shows a diagram of the layer structure of a sensor according to the invention . fig2 shows a side view of a vertical section through a sensor of fig1 along the line a -- a . fig3 shows the heating resistor , applied to aluminum oxide ceramic from fig1 in a view from below . reference numbers are used in fig1 to clarify the first aluminum oxide layer 1 and the second aluminum oxide layer 2 , the electric heating resistor 6 being only just visible on the underside of the second aluminum oxide layer 2 . adjoining the second aluminum oxide layer 2 there is the first zirconium dioxide layer 3 which on its top side carries an electrode 7 made of platinum . the zirconium dioxide layer 4 having the central cutout then follows , and the termination is finally formed by the zirconium dioxide layer 5 which on its top side carries two electrodes 8 and 8 &# 39 ;, arranged parallel to one another , one made of platinum and one made of a platinum alloy , which are coated , in the region of the actual measuring surface , with a thin protective layer 9 made of mullite , spinel or porous zirconium dioxide . in fig2 identical reference numbers have the identical meaning as in fig1 . fig3 shows the second aluminum oxide layer 2 in a view from below , illustrating , in particular , how the electric heating resistor layer 6 extends like a meander over the underside of the layer 2 .