EP0065806B1 - Voltage-dependent resistor and its manufacturing process - Google Patents

Description

The invention relates to a voltage-dependent resistor with a ceramic sintered body based on a polycrystalline, alkaline earth metal titanate doped with a small amount of a metal oxide to produce an N-type conductivity, with electrodes arranged on opposing surfaces, and a method for producing such a resistor.

A voltage-dependent resistor is known from German patent application P 30 19 969.0, which is based on N-doped strontium titanate, to which a small proportion of a lead germanate phase was added before sintering, which leads to the formation of insulating grain boundary layers in the polycrystalline structure of the sintered body. This known resistance is because of its relatively high field strength - a current density z. B. of about 3 mAlcm ² results only in fields of about 6 kV / cm - can only be used to a limited extent; he is e.g. B. Not suitable for modern semiconductor circuits that work with low voltages.

The invention has for its object to develop a voltage-dependent resistor according to the preamble of the claim and a method for its production such that a voltage-dependent resistor with a low field strength is obtained.

besteht, worin bedeuten :

• A = Erdalkalimetall ;
• SE = Seltenerdmetall;
• Me = Metall mit einer Wertigkeit von 5 oder mehr;
• 0.000 5 < x < Löslichkeitsgrenze in der Perowskitphase ;
• y = 0,001 bis 0,02

und daß der Sinterkörper an seinen Korngrenzen durch Eindiffusion eines Metalloxids oder mindestens einer Metalloxid-Verbindung gebildete Isolierschichten enthält, wobei das Metalloxid oder die Metalloxid-Verbindung einen niedrigeren Schmelzpunkt hat als die Perowskitphase, die polykristalline Perowskitphase an deren Kornrandschichten gut benetzt und bei bei Betrieb des Bauelementes auftretenden Feldstärken reversible Durchbruchserscheinungen zeigt.This object is achieved in that the sintered body made of an alkaline earth metal titanate with a perovskite structure of the general formula

consists of:

• A = alkaline earth metal;
• SE = rare earth metal;
• Me = metal with a valence of 5 or more;
• 0.000 5 <x <solubility limit in the perovskite phase;
• y = 0.001 to 0.02

and that the sintered body at its grain boundaries contains insulating layers formed by diffusion of a metal oxide or at least one metal oxide compound, the metal oxide or the metal oxide compound having a lower melting point than the perovskite phase, the polycrystalline perovskite phase well wetted on the grain edge layers and during operation of the Component occurring field strengths shows reversible breakthrough phenomena.

A method for producing a voltage-dependent resistor with a ceramic sintered body based on a polycrystalline, alkaline earth metal titanate doped with a small amount of a metal oxide to produce an N-type conductivity is characterized in that the sintered body with a perovskite structure is first produced in a reducing atmosphere by producing it Sintered body is then covered on its surface to form an insulating layer on the grain edge layers of the polycrystalline perovskite phase with a suspension containing at least one metal oxide or a metal oxide compound, the metal oxide or metal oxide compound having a lower melting point than the perovskite phase, the polycrystalline perovskite phase on the other Grain edge layers well wetted and showing reversible breakthrough phenomena during field strengths during operation of the component, after which the sintered body in an oxidizing atmosphere, preferably in air, b at a temperature which is above the melting point of the suspension component (s).

• I = Strom durch den Varistor in Ampere; U = Spannungsabfall am Varistor in Volt; C = geometrieabhängige Konstante ; sie gibt die Spannung an bei I = 1 A (in praktischen Fällen kann sie Werte zwischen 15 und einigen tausend annehmen) ; ß = Stromindex, Nichtlinearitätskoeffizient oder Regelfaktor. Er ist materialabhängig und ist ein Maß für die Steilheit der Strom-Spannungs-Kennlinie. Vorzugsweise soll der β-Wert so klein wie möglich sein, weil bei einem kleinen Wert für β starke Stromänderungen nur zu kleinen Spannungsänderungen am Varistor führen.

The voltage-dependent resistor according to the invention is characterized by a field strength that is lower by a factor of 10 compared to the known voltage-dependent resistor. Several factors are important for this: firstly, that the sintered body is produced under the influence of a slight TiO ₂ excess and, secondly, that it is diffused in by diffusion of a metal oxide which has a relatively low melting point in relation to the sintering body or one which has a relatively low melting point in relation to the sintered body Metal oxide compound has formed insulating layers. These insulating layers can have a gradient from the edge zone of the sintered body to the thickness of the sintered body. The Ti0 ₂ excess of the starting material for the sintered body leads not only to the sintering conditions, but essentially the sintering temperature and, in addition to the concentration of the doping, to grain growth. The grain size of the polycrystalline structure has a decisive influence on the field strength of the voltage-dependent resistor (hereinafter referred to as the varistor). In general, the smaller the grain size, the higher the field strength. This is a decisive advantage over the known voltage-dependent resistor, in which only a small grain growth is possible. However, it should be pointed out that if the threshold voltage is too low, the current index β of the varistor assumes increasingly unfavorable values. The current index β results from the formula U = C. I _ß , in which mean:

• I = current through the varistor in amperes; U = voltage drop across the varistor in volts; C = geometry-dependent constant; it indicates the voltage at I = 1 A (in practical cases it can take values between 15 and a few thousand); ß = current index, non-linearity coefficient or control factor. It depends on the material and is a measure of the steepness of the current-voltage characteristic. The β value should preferably be as small as possible, because with a small value for β, strong current changes only lead to small voltage changes at the varistor.

According to an advantageous development of the invention, the alkaline earth metal titanate is reacted by reacting SrC0 ₃ with Ti0 ₂ in a molar ratio of 1: 1.001 to 1: 1.02 with the addition of the doping metals in the form of their oxides in an amount of 0.05 to a maximum of 60 mol% of the constituent to be substituted after grinding and presintering for 15 h at 1,150 ° C. in air.

After grinding and granulating this sintered material and then pressing the ground material into a shaped body suitable for a resistor, according to a further advantageous embodiment of the invention, this is made from 90 for 4 h at a temperature of 1,460 ° C. in a reducing atmosphere consisting of mixed gas saturated with water vapor Vol.% N ₂ and 10 vol.% H ₂ sintered.

According to further advantageous embodiments of the invention, La ₂ 0 ₃ , Nb ₂ 0 ₅ or W0 ₃ are used as the doping metal oxide and Bi ₂ 0 ₃ as the metal oxide to be diffused in, or lead germanate Pb ₅ Ge ₃ O _{11 is} used as the metal oxide compound to be diffused in.

La ^{3 +} , Nb ^{s +} and W ⁶⁺ ions have proven to be particularly suitable for N doping. However, other dopings are also conceivable, e.g. B. other rare earth metal ions such as Sm ³⁺ or Y ³⁺ ; instead of Nb ⁵⁺ , Ta ⁵⁺ , As ⁵⁺ or Sb ⁵⁺ and instead of W ⁶⁺ , Mo ⁶⁺ and U ⁶⁺ can be used.

Depending on their ionic radius, the doping ions are installed either on Sr or Ti sites in the perovskite lattice. Relevant studies have shown that the large La ^{3 + -} ion (r _{La3 +} = 0.122 nm) is built into an Sr site (r _{sr2 +} = 0.127 nm). Analogous studies with PbTi0 ₃ showed that the smaller Nb ⁵⁺ -lon (r _{Nb5 +} . = 0.069 nm) is built into Ti sites (rr ₄ + = 0.064 nm). The W ⁶⁺ ion (rw., = 0.062 nm) can be assumed accordingly that it is also built into Ti sites.

Only when the sintered body is sintered in a reducing atmosphere, do the donor charges contribute directly to the conductivity. This state is called electron compensation. The chemical characterization of such electronically compensated, semiconducting perovskite samples with N-doping is for the doping of the present ceramic

( ^. = Symbol for donor electron). After the sintering, a suspension with at least one metal oxide that melts relatively low with respect to the sintered body or at least one metal oxide compound that melts relatively low with respect to the sintered body, e.g. B. Bi ₂ 0 ₃ or lead germanate Pb ₅ Ge ₃ O ₁₁ , applied in an organic binder and baked under oxidizing conditions at temperatures around or above 900 ° C, the applied molten metal oxide or the metal oxide compound diffuses preferably along the grain boundaries into the semiconducting ceramic and creates highly insulating grain boundary layers there.

• Figuren 1 und 2 Strom-Spannungs-Kennlinien von unterschiedlichen Varistoren gemäß der Erfindung ;
• Figur 3 Kurve der Temperaturabhängigkeit der Spannung über einem Varistor gemäß der Erfindung bei 1 mA und 30 mA.

Exemplary embodiments of the invention are described with reference to the drawing and their mode of operation is explained. Show it :

• Figures 1 and 2 current-voltage characteristics of different varistors according to the invention;
• Figure 3 curve of the temperature dependence of the voltage across a varistor according to the invention at 1 mA and 30 mA.

1 shows the current-voltage characteristic of a varistor of the composition Sr (Ti _0.996 W _0.004 ) O ₃ .0.01TiO ₂ and a diffused phase of Pb ₅ Ge ₃ O ₁₁ . The current density in mA / cm ^{2 is} plotted against the field strength over the component in kV / cm (thickness of the sintered body 400 μm; diameter of the sintered body 5 mm = 0.196 cm ² ). 1 shows that a current density of approximately 3 mA / cm ² is obtained even at relatively low fields of approximately 0.7 kV / cm. The varistor found is therefore distinguished from the known varistor by a field strength that is lower by a factor> 10. This makes the varistor in use particularly suitable for modern semiconductor circuits that operate at low voltages. A comparable behavior can also be found in Nb- and La-doped SrTi0 ₃ varistors according to the invention.

2 shows the current-voltage characteristic of a varistor of the composition Sr (Ti _0.996 W _0.004 ) O ₃ ^. 0.01TiO ₂ with a diffused phase of Bi ₂ O ₃ . The current in mA is plotted against the voltage in volts. The negative characteristic range begins at around 17 mA.

In Fig. 3, the voltage across a varistor of composition is Sr (Ti _0.996 W _0.004 ) O ₃ ^. 0.0TiO ₂ with a diffused phase of Bi ₂ O ₃ at 1 mA and 30 mA depending on the temperature. The sintered body of this varistor had a thickness of 400 μm and a diameter of 5 mm = 0.196 cm2.

The production of voltage-dependent resistors according to the invention is described below:

1. Production of the ceramic sintered body:

SrC0 ₃ , Ti0 ₂ and La ₂ O ₃ or Nb ₂ O ₅ or W0 ₃ as doping metal oxides were used as starting materials for the ceramic sintered body. When preparing the ceramic mass according to the compositions (Sr ₁ _ _x La _x ) TiO ₃ ^. yTiO _{2 '} Sr (T _1-X Nb _X ) O ₃ ^. yTi02 or Sr (T _1-X W _X ) O ₃ ^. yTiO ₂ with 0.000 5 <x <solubility limit in the perovskite phase and y = 0.001 to 0.02 should be observed that the Ti0 ₂ excess with 0.001 to 0.02 is chosen to always have a small excess of Ti ⁴⁺ ions. In this way, a liquid sintering phase is formed with the SrTi0 ₃ at a sintering above 1 400 ° C - it can be assumed that this is the eutectic SrTi0 ₃ -Ti0 ₂ occurring at ≈ 1 440 ° C, which is caused by the addition of Dopants can also occur at lower temperatures. A liquid sintering phase of this type promotes coarse grain growth, which, as already stated, is desirable.

The raw materials are weighed in an amount corresponding to the desired composition and 2 hours in a ball mill, e.g. B. agate, wet mixed. This is followed by presintering at 1,150 ° C for 15 h. The pre-sintered powders are ground again wet (1 h in a ball mill, e.g. made of agate). The millbase is then dried, and the powders thus obtained are then removed using a suitable binder, e.g. B. a 10% aqueous polyvinyl alcohol solution, granulated. The granules are molded articles suitable for ceramic resistors, for. B. pressed into disks with a diameter of - 6 mm and a thickness of 0.50 mm to a green density (bulk density) of about 55 to 60% of the theoretical density. This is followed by sintering the compacts at a temperature of 1 460 ° C for 4 h in a reducing atmosphere. The atmosphere can e.g. B. consist of mixed gas saturated with water vapor from 90 vol.% N ₂ and 10 vol.% H ₂ . Since the oxygen partial pressure of the mixed gas is determined by the ratio of the two partial pressures p _H2 / pHO, the mixed gas was saturated with H ₂ O at = 25 ° C in order to create an always comparable reduction atmosphere.

With regard to sintering, it is remarkable that coarse-grained structures preferably occur at sintering temperatures above 1,440 ° C. The reducing sintering should take place in a tightly closing furnace, e.g. B. a tube furnace is suitable. Excess reducing gas should expediently flow out via a bubble counter in order to create a constant sintering atmosphere.

Sintered bodies produced in this way are semiconducting and no longer show open porosity.

2. Production of the insulating layers on the grain edge regions of the polycrystalline perovskite phase:

The insulating grain edge layers are by diffusing at least one molten metal oxide or at least one metal oxide compound, for. B. Bi ₂ 0 ₃ or lead germanate Pb ₅ Ge ₃ O ₁₁ , generated in air. The metal oxide or the metal oxide compound is first suspended in a binder based on polyvinyl acetate and applied to the already sintered ceramic. The suspended metal oxide or the suspended metal oxide compound is then baked into the sintered body by a tempering process at a temperature at which they are in the molten state. For the metal oxide Bi ₂ 0 _{3 used} (melting point: ≈ 825 ° C) or the metal oxide compound Pb _s Ge ₃ 0 ₁₁ (melting point: ≈ 710 ° C), the minimum annealing temperature was a temperature slightly above the melting point of the metal oxide used or the used metal oxide compound determined. The amounts of the metal oxides or metal oxide compounds diffused into the sintered body were determined in parallel experiments by weighing the sintered bodies before applying the suspension, after burning out the binder in air at 600 ° C. and after tempering.

• a) bei einer festen Temperzeit von 120 min wurden jeweils unterschiedliche Sinterkörper auf Temperaturen von 900 °C, 1 000 °C, 1 100 °C, 1 200 °C und 1 300 °C erhitzt ;
• b) bei einer festgelegten Temperatur von 1100 °C wurden jeweils unterschiedliche Sinterkörper über eine Dauer von 5 min, 30 min, 60 min, 120 min und 240 min getempert ;
• c) die Sinterkörper wurden über eine Temperdauer von 120 min bei einer Tempertemperatur von 1 200 °C erhitzt (Standardbedingungen).

Annealing was carried out in different ways:

• a) at a fixed annealing time of 120 min, different sintered bodies were heated to temperatures of 900 ° C, 1,000 ° C, 1,100 ° C, 1,200 ° C and 1,300 ° C;
• b) at a specified temperature of 1100 ° C., different sintered bodies were annealed over a period of 5 min, 30 min, 60 min, 120 min and 240 min;
• c) the sintered bodies were heated over a tempering period of 120 min at a tempering temperature of 1200 ° C (standard conditions).

The heating-up and cooling-down times were uniformly 100 minutes for all tests.

3. Production of voltage-dependent resistors:

On the sintered body prepared as described, electrodes made of suitable metals, preferably gold, e.g. B. by vapor deposition. For better adhesion of the electrode metal, it is advisable to first apply a suitable adhesive layer as an intermediate layer between the ceramic and the electrode metal on the ceramic sintered body; e.g. B. a Cr-Ni layer is suitable.

• (Sr_1-XLa_X)TiO₃ · yTiO₂ (0,000 5 < x < Löslichkeitsgrenze des La in der Perowskitphase ; y = 0,001 bis 0,02) : wird x < 0,000 5, oxidieren die zu sinternden Körper zu schnell, die Reproduzierbarkeit der Resultate ist nicht mehr gewährleistet.

Notes on special compositions:

• (Sr _1-X La _X ) TiO ₃ · yTiO ₂ (0.000 5 <x <solubility limit of La in the perovskite phase; y = 0.001 to 0.02): if x <0.000 5, the bodies to be sintered oxidize too quickly, the Reproducibility of the results is no longer guaranteed.

The upper limit of x results from the solubility limit of La in the perovskite phase. Optimal Results were obtained with sintered bodies which had a microstructure with grains with a diameter of 80 to 120 microns with x = 0, _01, and y = 0.01 at a Sinterptemperatur of 1 460 ° C in a reducing atmosphere.

• für die Untergrenze von x gilt das gleiche, wie oben zu den La-Dotierungen ausgeführt ; ab x = 0,03 und mehr wurden homogene Mikrostrukturen nicht mehr reproduzierbar beobachtet. Optimale Ergebnisse wurden erreicht mit Sinterkörpern, die ein Gefüge mit Körnern eines Durchmessers von 60 bis 80 µm hatten mit x = 0,01 und y = 0,01 bei einer Sintertemperatur von 1 460 °C in reduzierender Atmosphäre.

Sr (Ti _1-X Nb _X ) O ₃ · yTiO ₂ (0.000 5 <x <solubility limit of the Nb in the perovskite phase; y = 0.001 to 0.02):

• the same applies to the lower limit of x as stated above for the La doping; From x = 0.03 and more, homogeneous microstructures were no longer reproducibly observed. Optimal results were achieved with sintered bodies that had a structure with grains with a diameter of 60 to 80 µm with x = 0.01 and y = 0.01 at a sintering temperature of 1 460 ° C in a reducing atmosphere.

• für die Untergrenze von x gilt das gleiche, wie oben zu den La-Dotierungen ausgeführt ; ab x ≈ 0,01 wurden überwiegend feinkörnigere Mikrostrukturen beobachtet, ab x ≈ 0,06 und mehr tritt zunehmend eine Ausscheidung von Fremdphasen in der Mikrostruktur auf, die aus SrW0₄ und TiO₂ besteht. Optimale Ergebnisse wurden erreicht mit Sinterkörpern, die ein Gefüge mit Körnern eines Durchmessers von 60 bis 80 µm hatten mit x = 0,004 und y = 0,01 bei einer Sintertemperatur von 1 460 °C in reduzierender Atmosphäre.

Sr (Ti _1-X W) O ₃ .yTiO ₂ (0.000 5 <x <solubility limit of W in the perovskite phase; y = 0.001 to 0.02):

• the same applies to the lower limit of x as stated above for the La doping; From x ≈ 0.01 predominantly more fine-grained microstructures were observed, from x ≈ 0.06 and more there is an increasing separation of foreign phases in the microstructure, which consists of SrW0 ₄ and TiO ₂ . Optimal results were achieved with sintered bodies that had a structure with grains with a diameter of 60 to 80 µm with x = 0.004 and y = 0.01 at a sintering temperature of 1 460 ° C in a reducing atmosphere.

4. Results

• Die nachfolgenden Tabellen 1 bis 3 zeigen die Ergebnisse der Eindiffusionsversuche mit aufgebrachten Suspensionen aus Bi₂0₃ und Pb₅Ge₃O₁₁. Die für die Eindiffusionsversuche verwendeten Sinterkörper hatten einen Durchmesser von 5 mm und eine Dicke von ca. 400 µm. Bei einer relativen Dichte der Sinterkörper von 97 bis 99 % der theoretischen Dichte betrug das durchschnittliche Gewicht eines Sinterkörpers 0,04 g. Die Menge des auf die Sinterkörper aufgebrachten Metalloxids oder der Metalloxid-Verbindung in Gew.%, bezogen auf das Gewicht des Sinterkörpers, wird als m₁ und die nach dem Tempern in der Keramik vorhandene Menge als m₂ bezeichnet.

Results of the diffusion tests:

• Tables 1 to 3 below show the results of the indiffusion tests with applied suspensions of Bi ₂ 0 ₃ and Pb ₅ Ge ₃ O ₁₁ . The sintered bodies used for the diffusion tests had a diameter of 5 mm and a thickness of approx. 400 µm. With a relative density of the sintered body of 97 to 99% of the theoretical density, the average weight of a sintered body was 0.04 g. The amount of the metal oxide or the metal oxide compound applied to the sintered body in% by weight, based on the weight of the sintered body, is designated as m ₁ and the amount present in the ceramic after annealing as m ₂ .

• Die Tabellen 1 bis 3 zeigen, daß alle Materialien, die eine Diffusionsphase aus Pb₅Ge₃O₁₁ hatten, brauchbare VDR-Effekte (VDR = voltage dependent resistor) zeigen, die sich gegenüber den Parametern der bekannten Varistoren um eine um einen Faktor > 10 niedrigere Einsatzfeldstärke bei in etwa gleichem Wert für den Stromindex β auszeichnen. Die'Tabelle 2 zeigt, daß Änderungen der Temperdauer und der Tempertemperatur keinen systematischen Einfluß auf die Werte für die Einsatzspannung und den Stromindex haben.

Results of the electrical measurements:

• Tables 1 to 3 show that all materials that had a diffusion phase from Pb ₅ Ge ₃ O _{11 show} usable VDR effects (VDR = voltage dependent resistor), which differ by a factor of> from the parameters of the known varistors 10 lower field strength with approximately the same value for the current index β. Table 2 shows that changes in the annealing time and the tempering temperature have no systematic influence on the values for the threshold voltage and the current index.

Different operating voltages of the finished component can, however, be set by different thickness of the components.

The sintered bodies treated with a diffusion phase made of Bi ₂₀₃ show the normal VDR dependence superimposed on a negative resistance range, ie, the voltage across the component decreases with increasing current, which can be advantageous in certain applications, since this is practically a value for the current index β <0 corresponds (for this, reference is made to FIG. 2). An overvoltage is thereby not only limited to a certain value, but additional energy is absorbed in the component as the current decreases as the current increases. This property of the sintered body treated with Bi ₂ 0 ₃ is only partially caused by the heating and the associated decrease in resistance of the components. This is shown in FIG. 3, in which the voltage across the component was plotted at 1 mA and 30 mA as a function of the temperature. The 30 mA values were measured by short current pulses, so that self-heating by the measuring current is negligible.

(See tables on page 6 ff.)

Claims (18)

1. A voltage-dependent resistor having a ceramic sintered body formed of a polycrystalline alkaline earth metal titanate doped with a small quantity of a metal oxide so as to produce an N-type conductivity having electrodes provided on oppositely located surfaces, characterized in that the sintered body comprises an alkaline earth metal titanate having a perovskite structure of the general formula

wherein

A = alkaline earth metal

SE = rare earth metal,

Me = metal having a valency of 5 or more

0.000 5 < x < solubility limit in the perovskite phase

y = 0.000 1 to 0.02

and that the sintered body comprises insulating layers at its grain boudaries, which are formed by indiffusion of at least a metal oxide or at least a metal oxide compound, which metal oxide or metal oxide compound has a lower melting point than the perovskite phase, which properly wets the grain boundary layers of the polycrystalline perovskite phase, and which exhibits reversible breakdown phenomena at field strengths occurring during operation of the component.

2. A voltage-dependent resistor as claimed in claim 1, characterized in that strontium is chosen as an alkaline earth metal.

3. A voltage-dependent resistor as claimed in claim 1, characterized in that lanthanum is chosen as a rare earth metal.

4. A voltage-dependent resistor as claimed in claim 1, characterized in that niobium is chosen as a metal having the valency 5.

5. A voltage-dependent resistor as claimed in claim 1, characterized in that tungsten is chosen as a metal having the valency > 5.

6. A voltage-dependent resistor as claimed in claim 1, characterized in that Bi₂0₃ is chosen as a metal oxide to be indiffused.

7. A voltage-dependent resistor as claimed in claim 1, characterized in that leadgermanate Pb₅Ge₃O₁₁ is chosen as a metal oxide compound to be indiffused.

8. A method of manufacturing a voltage-dependent resistor having a ceramic sintered body formed of a polycrystalline alkaline earth metal titanate doped with a small quantity of a metal oxide so as to produce an N-type conductivity, as claimed in any one of the preceding claims, characterized in that first the sintered body having a perovskite structure is manufactured in a reducing atmosphere, that subsequently, in order to form an insulating layer at the grain boundary layers of the polycrystalline perovskite phase, the surface of said sintered body is covered with a suspension comprising at least a metal oxide or a metal oxide compound, which metal oxide or metal oxide compound has a lower melting point than the perovskite phase, properly wets the grain boundary layers of the polycrystalline perovskite phase and exhibits reversible breakdown phenomena at field strengths occurring during operation of the component, after which the sintered body is tempered in an oxidizing atmosphere, preferably in air, at a temperature above the meltingpoint of the suspension component(s).

9. A method as claimed in claim 8, characterized by the following operational steps :

a) grinding a mixture of the starting substances for an alkaline earth metal titanate having perovskite structure with an addition of a metal oxide having a doping effect so as to produce an N-type conductivity according to the formula :

wherein :

A = alkaline earth metal

SE = rare earth metal

Me = metal having a valency of 5 or more 0.000 5 < x < solubility limit in the perovskite phase

y = 0.001 to 0.02 ;

b) pre-sintering the ground material according to step a) for 2 to 20 hours in the temperature range from 1 050 to 1 350 °C in air;

c) grinding and granulating the sintered material according to step b) with a suitable binder ;

d) compressing the ground material according to step c) to form a moulded body suitable for a resistor ;

e) sintering the moulded body according to step d) for 1 to 10 hours at a temperature in the range from 1 400 to 1 500 °C in a reducing atmosphere ;

f) providing the suspension containing the metal oxide(s) or the metal oxide compound(s) on the surface of the sintered body according to e) ;

g) indiffusing the suspension component(s) according to step f) into the sintered body at temperature above the melting temperature of the individual suspension component(s) in an oxidizing atmosphere, preferably in air;

h) providing metal electrodes on oppositely located surfaces of the sintered body according to step g).

10. A method as claimed in claim 9, characterized in that the alkaline earth metal titanate was formed by conversion of SrC0₃ with Ti0₂ in the molar ratio 1 : 1.001 to 1 : 1.02 while adding to doping metals in the form of their oxides in a quantity of 0.05 to at most 60 mol.% of the constituent to be substituted after grinding and presintering at 1 150 °C in air for 15 hours.

11. A method as claimed in claim 9, characterized in that La₂0₃ is used as a doping metal oxide.

12. A method as claimed in claim 9, characterized in that Nb₂0₅ is used as a doping metal oxide.

13. A method as claimed in claim 9, characterized in that W0₃ is used as a doping metal oxide.

14. A method as claimed in claim 9, characterized in that a 10 % aqueous polyvinyl alcohol solution is used as a binder.

15. A method as claimed in claim 9, characterized in that the moulded body according to step d) is sintered for 4 hours at a temperature of 1 460 °C in a reducing atmosphere consisting of a water vapour- saturated gaseous mixture of 90 % by volume of N₂ and 10 % by volume of H₂.

16. A method as claimed in claim 15, characterized in that the gaseous mixture is saturated with H₂0₂ at % 25 °C.

17. A method as claimed in claim 9, characterized in that Bi₂0₃ suspended in polyvinyl acetate solution is used as a metal oxide to be diffused according to step f).

18. A method as claimed in claim 9, characterized in that lead germanate Pb₅Ge₃O₁₁ suspended in polyvinyl acetate solution is used as a metal oxide compound to, be diffused according to step f).

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