DE2441450A1 - Semiconductor ceramic condenser of redn. re-oxidn. type - based on calcium-strontium-bismuth titanates for low loss and temp. dependence - Google Patents

Abstract

A semiconductor ceramic condenser of the redn. re-oxidn. type, with a substrate with plated Ag electrodes and connecting leads, has a substrate of an O-deficient semiconductor material based on a solid soln. of CaTiO3, SrTiO3 and a component of the formula Bi2O3.xTiO2 (in which 0.5 = x =9.0) contg. 0.025-0.4 wt. % Mn, Co, Ni, Cr, V, Nb, Ta, La and/or Ce ions and having a compsn. within the range defined by the following points A to D in the triangular diagram: (wt. %): CaTiO3, SrTiO3, Bi2O3 x TiO2: (A; 98, 0, 2), (B; 0, 98, 2), (C; 0, 50, 50), and (D; 50, 0, 50). The surface of the substrate forms a dielectric layer of the oxidn. type by treatment with the given amt. of O2. These condensers have a much lower loss tangent, lower temp. dependence of the loss tangent and capacity and higher rupture voltage than conventional semiconductor ceramic condensers.

Description

Semiconductor ceramic capacitor The invention relates to a semiconductor ceramic capacitor of the reduction-reoxidation type with a base body with plated-on silver electrodes and electrically connected lines.

In particular, the present invention is concerned with semiconductor ceramic capacitors from the SrTiO2-CaTiO3-Bi2O3 # xTiO2 type and especially with those capacitors which are used for are suitable for use in circuits which, with regard to their temperature dependence are subject to strict requirements, such as coupling circuits or catch circuits.

Although semiconductor ceramic capacitors are a relatively new development It has been found that they represent common capacitors with common ceramic are superior because they have a high capacity, small and relatively compact and have other excellent properties. ei the semiconductor ceramic capacitors A distinction is currently made between two types, namely the so-called valence-controlled Type (valence control type) and the oxidation-reduction type.

This distinction is made according to the composition of the Ceramic material and how the capacitors are made. The basic body semiconducting ceramics are predominantly made of the valence-controlled type Barium titanate, to which small amounts of other elements are mixed, make one have similar ionic radius as the components of barium titanate, however another valence. Because the properties of these semiconductors are valence-controlled Type strong on the purity of the psoh materials, the maintenance of this purity during manufacture and the accuracy of determining the proportions of each Depending on raw materials, it is difficult, if not impossible, to do so to manufacture semiconductor ceramics on an industrial scale. Indeed it is difficult to manufacture such ceramic materials in the laboratory, let alone in an industrial setting Scale.

Valence-controlled type semiconductors have additional disadvantages, there their specific resistance cannot be lowered below 10 Ohm # cm and because their electrical properties are inherently fixed, - so that the temperature dependence the capacity cannot be changed at will.

Capacitors with ceramic material are of the oxidation-reduction type free from the defects that are typical for valence-controlled ceramic materials, however, bring other Prc-ilemes with them. For example, it is shown in semiconductor ceramic capacitors of the oxidation-reduction type generally the subject that the insulation resistance drops sharply when the applied voltage is increased, so the working voltage in practice it is usually around 10 volts, while the upper limit for the voltage is set to around 12 olt. Another disadvantage of these Vcapacitors means that undesirable changes in the electrical properties occur, prnn the leads are soldered directly to the silver electrodes as it is in this Case is difficult to stabilize the boundary layers. To prevent this the leads are usually made with the help of a conductive adhesive attached to the silver electrodes. In practice, however, home soldering the condensers in a circuit, their leads to an elevated temperature er.r.; rmt, which in some cases can damage the conductive adhesive bond leads. Especially in miniaturized electrical circuits in which the 7uleitunqen are short, very careful handling is required.

In summary, it can be stated that the previously known Ceramic capacitors with barium titanate (BaTiO3) as the main component, the electrical properties are not affected of barium titanate, i.e. its temperature dependence with regard to the Capacity and the tan # and also not its high dielectric constant.

The use of the known capacitors is also due to the low Breakdown voltage of the same limited, as already mentioned above.

The invention is based on the prior art described the underlying task of proposing semiconductor ceramic capacitors which are free of the disadvantages indicated above and which are considerably smaller Have loss angle tan cf, which has a reduced temperature dependence of the Loss angle tan # and capacitance and which have a higher breakdown voltage than have the well-known semiconductor ceramic capacitors.

According to the invention, this object is achieved by a semiconductor ceramic capacitor solved the type described above, which is characterized in that the base body is made of a semiconducting material that has an oxygen deficiency consists, which consists essentially of a solid solution of calcium titanate, strontium titanate and a component, for which the formula applies: Bi2O3 # xTiO2 where for x the following relationship holds true: 0.5 # x # 9.0 that the basic body Ions contains at least one of the following group of substances: manganese, cobalt, Mickel, chromium, vanadium, niobium, tantalum, lanthanum, and zer in a total amount between 0.025 and 0.4 percent by weight that the composition of the solid solution falls into the square area with the corner points A to D in Fig. G, where for the points A to D the compositions given in the following table apply: Point CaTiO3 SrTiO3 Bi2O3 # xTiO2 (wt%) (wt%) (wt%) A 98.0 0 2.0 B 0 98.0 9, n C O 5n, 0 5n, 0 D 50.0 n 50.0 and that the surface of the base body by treatment with a predetermined amount of oxygen, an oxidation type dielectric layer forms.

According to the present invention, the semiconductor ceramic capacitors preferably made from mixtures, which predominantly from non-stoichiometric There are proportions of solid solutions obtained from a mixture contained in the mainly made of strontium titanate (SrTiO3), calcium titanate (CaTiO3), bismuth oxide (Bi2O3) and titanium oxide (TiO2) are composed and which smaller proportions are more suitable Quantities of ions of at least one of the following substances are added: Manganese, Cobalt, nickel, chromium, vanadium, niobium, tantalum, lanthanum and Zcr.

This mixture is then heated in an oxidizing atmosphere, to create a ceramic material. The ceramic material is then in reducing Atmosphere is heated, and it gets some amount of the oxygen out of the ceramic material removed to keep the non-stoichiometric solid solutions. Afterward the non-stoichiometric solid solutions are partly at elevated temperature partially reoxidized in an oxidizing atmosphere to make semiconductor ceramic capacitors to obtain.

r.s is an advantage of the semiconductor capacitors according to the invention, that they with regard to the bias voltage that can be applied to them, with regard to their frequency characteristics and with regard to the aging of the capacitance characteristic and the loss angle tan # have excellent properties.

Another advantage of the semiconductor ceramic capacitors according to the invention consists in the fact that they have capacitor plates with stable boundary layers, which are obtained due to a reduction-reoxidation process, so that the Leads can be soldered directly to the silver electrodes without a Deterioration of any of the electrical properties occurs and without damage to the connection area on the electrodes occurs. This means, that the properties of the capacitors are improved compared to a thermal shock be, so that a remarkable improvement in the labor output in the manufacture the Capacitors and when installing them in electrical circuits can be reached.

I3ei the known processes for the production of semiconducting ceramics it is necessary to use extremely pure raw materials, which reduces manufacturing costs however, can be increased.

According to the present invention, however, semiconductor ceramic capacitors have been used developed, which have excellent electrical properties, but which considerably cheaper to manufacture using common industrial raw materials are.

The semiconductor ceramic capacitors according to the invention can according to can be produced using the following process: SrTiO3, CaTiO3, Bi2O3, TiO2 and the ions one or more of the following elements: Mn, Co, Ni, Cr, V, Nb, Ta, La and Ce are weighed in such amounts that the desired composition results. The ingredients of the mixture are then placed in a china pot or drum given and mixed together.

The raw material mixture obtained is then coated with binders mixed. The mixture obtained in this way is pressed and made into slices or the like shaped. The molded base bodies are sintered in an oxidizing atmosphere, to get ceramics. Then the ceramic bodies are again in a reducing Atmosphere heated. During this process step, the ceramic bodies lose a Part of the oxygen and T, 7 earths semiconducting. The semiconducting Ceramic grains obtained in this way are then grounded with silver electrodes provided and heated in an oxidizing atmosphere. This final heat treatment serves to plate the silver electrodes, the surface diffusion of the electrode material on the base body and at the same time the partial oxidation of the ceramic base body.

In this way, semiconductor ceramic capacitors are obtained.

All of the manufacturing process steps described above of semiconductor ceramic capacitors according to the invention are essential to the specified features and properties of the ceramic base body.

The raw materials useful in connection with the present invention can be oxides or compounds, which result in oxides when heated, such as z.ß. Carbonates, nitrates, hydroxides, hydrates, etc.

The manganese ions can, for example, in the form of manganese carbonate, sulfate, nitrate and hydroxide are added or in the form of the manganese salts of organic acids.

The invention is explained in more detail below with reference to a drawing. The drawings show: FIG. 1 a graphic representation of the change in the capacitance and the loss angle tan # of a semiconductor ceramic capacitor according to the invention depending on the applied DC voltage; Fig. 2 shows the dependency of the loss angle tan # from the frequency; Fig. 3 is a graphic representation of the Influence of aging on the hzw. the loss angle tan #; Fig. 4 a graphical representation of the temperature dependence of the capacitance and the loss angle; Fig. 5 is a graphical representation of the dependence of the change in capacitance, the percentage change in ranacite cite and the loss angle from the value x of the Bi2O3 xTiO2 component and FIG. 6 shows a triangular mixture diagram of the internal system CaTiO3-SrTiO3-Bi2O3 # 3TiO2, in what contour lines for the capacitance, the change in capacitance, based on the Kanazitat at 200C, and the loss angle are shown.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The raw materials are put together and mixed in such a way that the following mixture results: SrTiO3 66.0 percent by weight, CaTiO3 27.0 percent by weight, Bi2O3 # 3TiO2 7.0 percent by weight and manganese ions 0.05 percent by weight. This mixture comes in a wet state Porcelain clay mixed using agate balls. The SrTi03 and the Be CaTiO3 before mixing with strontium carbonate (SrCO3) and Titanium dioxide (TiO2) or calcium carbonate (CaCO3) and titanium dioxide (TiO2) prepared, by burning the equimolar mixture at a temperature of 1200 ° C and the The burnt mixture is then roughly chopped up.

The manganese ions are added in the form of manganese carbonate.

After mixing in the wet state, the ßTix L3indmittel added, and there is sufficient mixing in a ball mill. The dahei The resulting mixture is then pressed and made into disks with a diameter of 16.5 mm and a thickness of 0.6 mm, with a pressure of 3 t / cm2 is made.

The molded discs are kept at one temperature for 2 hours of 13500C in air and then sintered for 1 hour at a temperature of 8500 of a reducing atmosphere such as a hydrogen flow exposed. The semiconductor wafers lose during the last-mentioned process step some of the oxygen and become semiconducting. Then both surfaces are or the two main surfaces of the ceramic disks or base bodies obtained in this way smeared with a silver electrode paste.

The ceramic base bodies are then placed in an oxidizing atmosphere, for example heated in air to a temperature of 7800C to achieve a partial To achieve reoxidation of the ceramic body and therefore the painted material for the silver electrodes to be plated on the main surfaces of the ceramic body. The supply lines are then directly connected to the surface of the common electrodes, immersing them in the molten solder will. In this way, a semiconductor ceramic capacitor is obtained with the following characteristic values: capacitance (C) 0.04 µF / cm2 loss angle tan # 0.5% change in capacitance in the range between -55 and + 1200C (based on the capacitance at 25 ° C; measuring frequency 1 kHz) + 6.2% to -5.6% insulation resistance (IR) (measured at 25 C and 100 V) 5 x 10 M # breakdown voltage 800 V = In the above measurement results, the Measurement values obtained 24 hours after the reoxidation as initial values assumed and both the capacitance C and the loss angle tan # were with a frequency of 1 kHz, a voltage of 1 V and a temperature of 250C measured.

The change in the capacitance and the loss angle of the semiconductor ceramic capacitors as a function of the applied DC voltage is shown in FIG. 1. The frequency dependence of the loss angle tan # of the semiconductor ceramic capacitors is shown in FIG. Furthermore, the influence of aging on the capacity and the loss angle of the Semiconductor ceramic capacitors shown in Fig. 3 where along the zonzontal axis the time is plotted in hours.

From Fig. 1 it is clear that the capacitance and the loss angle not are dependent on the applied DC voltage.

As FIG. 2 shows, the stored amount of charge @ decreases in a frequency range from 1 kHZ to 100 MHz not below the value 100, i.e. the loss angle tan # is a maximum of 1%.

Furthermore, it is clear from FIG. 3 that even after a time of 1000 Hours the capacity and the loss angle are still identical to the initial values these sizes are.

Table II shows the electrical properties of common semiconductor ceramic capacitors of the reduction-reoxidation type with BaTiO3 and the corresponding electrical properties summarized by semiconductor ceramic capacitors according to the invention.

TABLE II No. Capacitor- C tan # Isol. V = T.C. Voltage type (µF / (%) Wid. (%) characteristic cm2 (M #) # C (%) 1 BaTiO3-0.18 5.4 2 x 500 -8 - -40 Bi2O3 103 -25 BaTiO3-2 0.17 2.0 1 x 1200 -29- -40 105 -67 according to 0.04 0.5 5 x 800 +4 - 0 3 4 Invention 10 -1 T.C. in the table above means a value calculated according to the following equation: T.C. (%) = # C / C20 x 100, where 4 C equals the difference between the capacitance at -300C and at +85 0C and where C20 is the Kanazitat bai 200C.

Fig. 4 shows the temperature dependency of the capacitance C and the loss angle tan # for capacitor types 1 to 3, according to table II. 4 the percentage change in capacity (# C%) in solid lines, and the loss angle tan # drawn in dashed lines. The numerals 1 to 3 in FIG. 4 denote the respective capacitor type according to Tahelle II. From Tables II and 4 it is clear that semiconductor ceramic capacitors according to the invention with regard to the loss angle tan # and the value T.C. the usual semiconductor ceramic capacitors are far superior. It is also clear from Table II and FIGS. 1 to 3, that the voltage characteristic, the frequency characteristic and the aging properties of semiconductor ceramic capacitors according to the invention are excellent.

It is clear from the above that the semiconductor ceramic capacitors according to the invention do not have any of the disadvantages associated with conventional semiconductor ceramic capacitors of the reduction-reoxidation type with BaTiO3 are present. In addition, they are according to the invention Semiconductor ceramic capacitors with regard to the temperature, the voltage, the Frequency and aging characteristics are very stable and can therefore be used with advantage be used in electronic circuits.

The T values given in Table II for capacitor type No. 1 were averaged at a voltage of 0.5 7 V and a frequency of 1 kHz, and for the loss angle and the value T.C. The insulation resistance was at measured at a voltage of 25 V. In Table II, the voltage characteristic means also the speed of the change in capacitance when a DC voltage is applied of 25 T. For capacitor type no. 2, the values for the loss angle became tan # and the value T.C. determined at a voltage of 1 V and a frequency of 1 kHz. The insulation resistance was determined at a voltage of 5n V. As a voltage characteristic is the rate of change in capacitance when a DC voltage is applied of 50 V. The values for capacitor type No. 3 in Table II were determined under the same conditions as for capacitor type no.1.

The following table III shows the electrical properties for different amounts of Mn ions in the mixture given above. this table thus provides information about the effect of the admixture of Mn ions.

TABLE III cond. Mn ions C tan # isol. Resist. V type (wt .-%) (µF / cm2) (%) (M #) (V =) No.

4 n 0.040 21 1 x 104 250 5 0.025 0.044 0.7 1 x 104 600 6 0.05 0.040 0.5 5 x 104 800 7 0.15 0.034 0.4 5 x 104 1200 8 0.40 0.021 a, 4 7 x 10 1600 9 0.60 0.0007 0.1 # 7000 As can be seen from Table III, the addition of Mn ions show a rapid increase in insulation resistance even in small quantities with himself. This increase in the insulation resistance is caused by the lowering of the Density of free electrons reached in the boundary layer, which is based on the valence Compensation effect of the Mn ions in the boundary layer is due.

For the capacitor types according to Table III, manganese carbonate (MnCO3) used as a source for the Mn ions and added in such an amount that gave a predetermined content of Mn ions.

When the content of Mn ions is below 0.025 percent by weight, show the ceramic base bodies that are produced from such a mixture, spots and pinholes on their surface and have no fine surfaces. Also show Capacitors made from such mixtures large fluctuations in their characteristic values. If on the other hand the proportion of Mn ions exceeds 0.60% by weight, the capacity becomes very large small, while the insulation resistance is very high. Dics is due to that the phenomenon of reoxidation is very violent during the reoxidation step and leads to a deterioration in the semiconductor ceramic capacitors.

The above-described effects of adding Mn ions are the same throughout the area A-B-C-D in FIG.

The amount of the addition of Mn ions is consequently to values between 0.025 and .40 percent by weight.

Other experiments have shown that the addition of CO-, Ni-, Cr-, V, Nb, Ta, La, and Ce ions instead of Mn ions give exactly the same results leads, which were obtained with Mn ions, and that the common re-mixing of two or more of the above ion types also make good semiconductor ceramic capacitors the characteristics of which are the same as those of the above considered semiconductor ceramic capacitors, provided that the total amount of admixed ions within the range of 0.025 to 0.40 percent by weight lies.

Fig. 5 shows a graphical representation of the relationship between the capacitance C, the percentage change in capacitance SC / C20 at a temperature of -30 ° C and the loss angle tan 6 on the one hand and the t1: ert x of the mixture component Bi2O3 # xTiO2, which is one of the main components of the ceramic material for Semiconductor ceramic capacitors according to the invention, on the other hand.

As long as x is below 0.5, the proportion of Bi2O3 is excessively large and the mixture falls out of the range given above, since the Bi2O3 during the heating evaporates, which is undesirably high for practical use of the individual properties. On the other hand, if x is greater than 9.0, there is an excess of TiO2 and the ceramic grains adhere to one another.

If x is between 0.5 and 9.0, as shown in FIG. 5, it will go sic, on the other hand, good values for the capacitance for the semiconductor ceramic capacitors, for the value T.C. and for the loss angle tan #. For the mix component Bi2O3 # xTiO2 are therefore preferred for x values between 0.5 and 9.0.

Otherwise, FIG. 5 applies with the different values of x for the P component Bi2O3 # xTiO2 for a ceramic mixture with 66.0 percent by weight SrTiO3, with 27.0 percent by weight CaTiO3, with 7.0 percent by weight Bi2O3 # xTiO2 and with 0.05 percent by weight of Mn ions.

Fig. 6 shows the contour lines of the capacitance C, the value T.C.

= @ C / C20 (%) and the loss angle tan # of semiconductor ceramic capacitors from the mixture given above with different proportions of the three compounds SrTiO3, CaTiO3 and Bi2O3 # 3TiO2.

As shown in FIG. 6, they have semiconductor ceramic capacitors excellent Properties with regard to the value T.C. and the loss angle tan #. These excellent Properties can be achieved using ceramic mixes that fall into the square area A-B-C-D in Fig. 6 and when considering these mixtures 0.025 to 0.4 percent by weight of ions of at least one of the following elements admixed: manganese, cobalt, nickel, chromium, vanadium, niobium, tantalum, lanthanum and cerium. Semiconductor ceramic capacitors with excellent properties can therefore be made from Ceramic mixtures are obtained that fall into the quadrangular area A-B-C-D, where for points A, B, C and D the mixture ingredients given in Table IV below can be used to which ions at least one of the following elements are mixed become: Mn, Co, Ni, Cr,, Nb, Ta, La and Ce.

TABLE IV Point CaTiO3 SrTiO3 Ri2O3 xTiO2 (% by weight) (% by weight) (% by weight) A 98.0 0 2.0 B 0 98.0 2.0 C 0 50.0 50.0 D 50.0 0 50.0 In Fig. 6 the value x is ftir Bi2O3 # xTiO2 = 3. However, this value is only a significant example. It will namely for Values of x in the range from 0.5 to 9.0 for semiconductor ceramic capacitors according to the invention achieved the same excellent properties as for the value x = 3, as is clear from FIG.

In the event that the proportion of Bi2O3 # xTiO2 in the Keramil basic bodies for capacitors according to the invention is below 2.0 percent by weight, it is difficult to release the oxygen from the ceramic base bodies, even if you have them exposed to a heat treatment in a reducing atmosphere, so that such Ceramic base body does not become semiconducting. This shows that decreased tight of bismuth hinder easy oxygen release and reoxidation. On the other hand, when the content of the Bi 203 xTiO2 becomes higher than 50.0% by weight it is difficult to sinter the formed base body, which is in practice proves unfavorable.

Accordingly, a proportion of Bi2O3 # xTiO2 is preferably between 2.0 and 50.0 percent by weight worked. The proportion of CaTi03 and SrTiO3 corresponds to the remainder of the proportion of Bi2O3 # xTiO2 seen. The proportion in other words, the CaTiO3 is between 0 and 98.0 percent by weight. if the proportion of CaTiO3 is 0 percent by weight, the proportion of SrTiO3 is up to 98.0 percent by weight, while if the proportion of CaTiO3 is 98.0 percent by weight is, the proportion of SrTiO3 is 0 percent by weight.

CaTiO3 and SrTiO3 are each in such a weight ratio that used to have the properties you want for C, tanrf and T.C. result.

In the event that the proportion of SrTi3 and CaTiO3 in the total mixture is below 50.0% by weight, sintering becomes difficult because the proportion is too high is present at Bi2O3 # xTiO2. On the other hand, the proportion of SrTin3 and CaTiO3 is 98.0 Exceeds the weight percent of the total mixture, there is a proportion of Bi203 xTiO2 less than 2.0 percent by weight, so that it becomes difficult in the ceramic base bodies contained oxygen by a heat treatment in a reducing atmosphere to release; so that the ceramic basic grains do not become semiconducting.

As described above in the individual articles the semiconductor ceramic capacitors, which by sintering the mixture according to the present Cr detection in an oxidizing atmosphere and subsequent heat treatment in a reducing atmosphere to release some of the oxygen the ceramic, basic bodies were prepared and to bring about semiconductor properties # Brushed with silver electrode paste and heated in an oxidizing atmosphere at the same time a plating of the silver electrodes, a surface diffusion of the electrode material and a partial reoxidation of the surface of the ceramic To achieve basic body and due to this treatment a smaller loss angle tan #, a lower temperature dependence of the capacitance and a higher insulation resistance in relation to the applied voltage than with previously known capacitors. Semiconductor ceramic capacitors according to of the invention can consequently are used in many fields, especially in circuits in which work is carried out with high voltages.

Semiconductor ceramic capacitors according to the invention are also praised none of the table parts that are common with the usual semiconductor ceramic capacitors from Reduction-reoxidation type with BaTiO3 are present and are advantageous in that than they have good voltage and frequency characteristics, with their electrical properties should not be influenced by aging. Consequently, semiconductor ceramic capacitors earth according to the invention also used in electronic circuits.

It also improves the ability to direct the leads to be able to solder to the surface of the electrodes, not only the near workability in the production of the semiconductor ceramic capacitors and in the installation of the same in electronic circuits, but also leads to good performance and simplification of production when such semiconductor ceramic capacitors in miniaturized electrical circuits are used.

Claims (2)

P a t e n t a n s p r ü c h e: Reduction-reoxidation type semiconductor ceramic capacitor having a Base body with plated-on silver electrodes and electrically connected to them Supply lines, characterized in that the base body consists of an oxygen deficiency having semiconducting material, which consists essentially of a solid Solution of calcium titanate, strontium titanate and a portion for which the formula applies: Bi2O3 # xTiO2 where the following relationship applies to x: 0.5 # x # 9.0 that the base body contains ions of at least one of the following group of substances: rtangan, cobalt, nickel ,. Chromium, vanadium, niobium, tantalum, lanthanum and cerium, all in one Total amount between 0.025 and 0.4 percent by weight that the composition of the solid solution falls into the quadrangular area with the corner points A to fl in Fig. 6, where for points A to D the compositions given in the following table are valid; Point CaTiO3 SrTiO3 Bi2O3 # xTiO2 (wt%) (wt%) (wt%) A 98.0 0 2.0 B O 98.0 2.0 C n 5n, 0 50.0 D 50.0 0 50.0 and that the surface of the Base body a dielectric by treatment with a predetermined amount of oxygen Forms oxidation type layer. 2) capacitor according to claim 1, characterized in that the base body is made from the following mixture of raw materials. SrTiO3 66.0% by weight CaTiO3 27.0% by weight Bi2O3 # 3TiO2 7.0% by weight Mn 0.05 Wt% 3) Method of manufacturing a semiconductor ceramic base body for a Capacitor according to claims 1 and 2, characterized by the following process steps: a) Strontium titanate, Italziumtitanat, bismutoxid and titanium oxide are in such Mixed proportions that result in a composition that is in the quadrangular Area A-B'-C-D in Fig. 6 falls and this composition will be Ions mixed with at least one of the following substances: manganese, cobalt, nickel, Chromium, vanadium, niobium, tantalum, lanthanum and cerium, in amounts between 0.025 and 0.4 percent by weight. b) The mixture obtained in process step a) becomes the desired one Basic bodies formed; c) earth the base bodies obtained in this way for the duration of about 2 hours at a temperature of about 1350 in an oxidizing atmosphere sintered; d) the sintered base bodies obtained in process step c) are for a period of about 1 hour in a reducing atmosphere to one temperature heated to about 850 ° C and e) the sintered obtained in process step d) Basic bodies are for a period of about 1 hour in an oxidizing atmosphere heated to a temperature of ctTra 780 ° C. L e r s e i t e