Non-linear resistor and method of manufacturing the same

Abstract

A non-linear resistor having an operational field strength which optionally is formed as a VDR- or as an NTC-resistor having a ceramic sintered body on the basis of a polycrystalline alkaline earth metal titanate doped with a small quantity of a metal oxide so as to produce an N-type conductivity, in which the sintered body comprises at its grain boundaries insulating layers formed by re-oxidation of the sintered body and consists of an alkaline earth metal titanate having a Perowskite structure of the general formula(A.sub.1-x Ln.sub.x)TiO.sub.3.yTiO.sub.2 or A(Ti.sub.1-x Me.sub.x)O.sub.3.yTiO.sub.2,wherein: A=alkaline earth metal; Ln=rare earth metal; Me=metal having a valency of 5 or more; 0.0005<.times.<solubility limit in the Perowskite phase; y.times.0.001 to 0.02. The sintered body becomes adjustable in its non-linear resistance variation by selection of the re-oxidation temperature and of the re-oxidation duration in such manner that an initially present NTC-characteristic gradually is observable only at ever increasing temperatures and changes into a VDR-characteristic in the range of the operating temperature of the resistor.

Description

The invention relates to a non-linear resistor having a ceramic sintered body on the basis of a polycrystalline alkaline earth metal titanate doped with a metal oxide to produce an N-type conductivity, the body having electrodes provided on oppositely located surfaces. The invention furthermore relates to a method of manufacturing such a resistor.

Non-linear resistors are to be understood to mean in this case resistors having an NTC-characteristic (resistance value decreases independently of the applied voltage with increase in temperature) and resistors having a VDR-characteristic (resistance value depends only on the applied voltage).

From U.S. patent application Ser. No. 263,321 (=published EP-PA No. 40,881) a voltage-dependent resistor is known which is based on N-doped strontium titanate to which prior to sintering a small quantity of a lead germanate phase was added which leads to the formation of insulating grain boundary layers in the polycrystalline grain texture of the sintered body. Due to its comparatively high operational field strength--a current density, for example, of approximately 3 mA/cm.sup.2 is obtained only with fields of approximately 6 kV/cm--this known resistor has only a limited field of application; for example, it is not suitable for modern semiconductor switching circuits operating at low voltages.

It is the object of the invention to provide a non-linear resistor as mentioned above and a method of manufacturing same in such a manner that not only a non-linear resistor having a low operational field strength is obtained, but that said resistor is formed optionally as a VDR or as an NTC-resistor.

According to the invention this object is achieved when the sintered body comprises at its grain boundaries insulating layers formed by re-oxidation of the sintered body and consists of an alkaline earth metal titanate containing excess TiO.sub.2 and having a Perowskite structure and a composition defined by one of the general formulae

(A.sub.1-x Ln.sub.x)TiO.sub.3.yTiO.sub.2 or A(Ti.sub.1-x Me.sub.x)O.sub.3.yTiO.sub.2 in which: A=alkaline earth metal Ln=rare earth metal, including yttrium Me=metal having a valency of 5 or more 0.0005&lt;x&lt; solubility limit in the Perowskite phase y=0.001 to 0.02.

A method of manufacturing the sintered body of the above-mentioned type is carried out according to the invention so that the ceramic body is first sintered in a reducing atmosphere and that said sintered body is then re-oxidized in an oxidizing atmosphere, preferably in air, in which the sintered body, by choice of the reoxidation temperature and the reoxidation duration, is adjustable in its non-linear resistance variation such that an initially present NTC-characteristic as a function of the oxidation state gradually can be observed only at increasing temperatures and, in the range of the operating temperature of the resistor, changes into a VDR-characteristic.

As a result of the sintering in a reducing atmosphere the sintered body is made continuously semiconductive and subsequently grain boundary layers of the semiconductor grains of the polycrystalline grain structure of the sintered body are converted by the formation of high-ohmic oxide layers by re-oxidation. In accordance with the value of the reoxidation temperature and the reoxidation duration, sintered bodies can thus be manufactured at will in which the NTC-characteristic or the VDR-characteristic predominates.

According to advantageous modified embodiments of the invention, strontium is chosen as an alkaline earth metal and La.sub.2 O.sub.3, Nb.sub.2 O.sub.5 or WO.sub.3 are chosen as doping metal oxides. The incorporation of the doping metal oxide in the Perowskite lattice of the SrTiO.sub.3 occurs by reaction already during the pre-sintering in the manufacture of the sintered body. In addition to the said dopants, other metal oxides are also feasible, for example, Y.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Ta.sub.2 O.sub.5, As.sub.2 O.sub.5, Sb.sub.2 O.sub.5, MoO.sub.3 or U.sub.3 O.sub.8.

In accordance with the ion radius the doping ions are incorporated either in Sr-sites or in Ti-sites in the Perowskite lattice of the SrTiO.sub.3. It has been demonstrated by means of X-ray structure analysis that the large La.sup.3+ -ion (r.sub.La 3+=0.122 nm) is incorporated in Sr-sites (r.sub.Sr 2+=0.127 nm). By analogous studies on PbTiO.sub.3 it could be demonstrated that the smaller Nb.sup.5+ -ion (r.sub.Nb 5+=0.069 nm) is incorporated in Ti-sites (r.sub.Ti 4+=0.064 nm). On the basis of the ion radius of the W.sup.6+ -ion (r.sub.W 6+=0.062 nm) it can accordingly be concluded that it is also incorporated in Ti-sites.

After the presintering the doped alkaline earth metal titanate is brought into a sinterable finely pulverized state by grinding in a ball mill and is formed into usually a disc-shaped body by compression. Only when sintering is carried out in a reducing atmosphere do the donor charges contribute directly to the conductivity. This condition is referred to as electron compensation. The chemical characterization of such electron-compensated, semiconductor Perowskite phases with n-doping follows for the dopings according to the invention: ##EQU1##

The electron-compensated materials have a resistivity in the order of magnitude of 1 .OMEGA.cm.

When on the contrary the samples are sintered in an oxidizing atmosphere, the compensation of the donor charges occurs via cation vacancies, mainly vacancies in Sr-sites. Such vacancy-compensated materials are highly insulating since the cation vacancies operate as very strong electron acceptors. The chemical characterization of vacancy-compensated materials are as follows for the dopings according to the invention:

Sr.sub.1-3x/2 La.sub.x.sup..multidot. .quadrature.".sub.x/2 TiO Sr.sub.1-x/2 .DELTA.".sub.x/2 (Ti.sub.1-x Nb.sub.x.sup..multidot.)O.sub.3 Sr.sub.1-x .DELTA..sub.x "(Ti.sub.1-x W.sub.x.sup...)O.sub.3 '=symbol for acceptor electron .quadrature.=symbol for lattice vacancies

These vacancy-compensated materials have a resistivity in the order of magnitude of 10.sup.13 .OMEGA..cm.

The invention is based on the recognition of the fact that the electron-compensated semiconductor ceramic can be converted into the highly insulating vacancy-compensated form by re-oxidation. A variety of transition states can be produced between the purely electron-compensated form which corresponds to a Perowskite ceramic having NTC-properties, and the purely vacancy-compensated form which corresponds to a Perowskite ceramic having VDR-properties. Reaction kinetic experiments on electron-compensated semiconductive Ba.sub.1-x La.sub.x .sup.. TiO.sub.3 (z.apprxeq.0.005 to 0.02) have demonstrated that the oxidation always begins at the grain boundaries and a semiconductor ceramic material having highly insulating grain boundary layers is formed. Analogous processes occur in the oxidation of semiconductor N-doped SrTiO.sub.3.

A particular advantage which is obtained with the invention is the low operational field strength in resistors having VDR-characteristic in addition to the adjustability of the characteristic of the resistors comprising a ceramic sintered body according to the present Application. As compared with the known resistor according to U.S. patent application Ser. No. 263,321, now U.S. Pat. No. 4,417,227, the resistors having VDR-characteristic according to the present Application are distinguished by an operational field strangth which is lower by a factor of .ltoreq.20.

As a result of this, varistors with sintered bodies according to the present invention become particularly applicable for modern semiconductor switching circuits operating at low voltages. The varistors having sintered bodies with La-doping, Nb-doping or W-doping all show the said low operational field strength. For this it is important for the sintered body to have a low TiO.sub.2 -excess and to comprise insulating layers formed by re-oxidation. These insulating layers may show a gradient in the resistivity from the edge zones of the sintered body over the thickness of the sintered body.

Grain growth of the titanate starting material during sintering depends upon the presence of an excess of TiO.sub.2 the dopant concentration and the sintering conditions particularly the sintering temperature. The grain size of the polycrystalline structure has a decisive influence on the operational field strength of the non-linear resistor. The smaller the grain size, the higher is in general, the operational field strength of the resistor.

It should be noted, however, that when the operational field strength is too low the current index .beta. assumes increasingly unfavourable values. The current index .beta. appears from the formula U=C . I.sup..beta., where I=current through the resistor in amperes; U=the voltage drop across the resistor in volts; C=a geometry-dependent constant, it indicates the voltage at I=1 A (in practical cases it may assume values between 15 and a few thousand); .beta.=current index, coefficient on non-linearity or control factor which is material-dependent and is a measure of the steepness of the current-voltage characteristic. Preferably the .beta. value should be as small as possible, because at a small value of .beta. large current variations lead only to small voltage variations across the non-linear resistor.

Embodiments of the invention and their operation will now be described in greater detail with reference to the drawings, in which

FIG. 1 shows a current-voltage characteristic of a non-linear resistor according to the invention,

FIGS. 2a to 2c show the dependence of the operational voltage with a current of 1 mA and of the current index .beta. on the re-oxidation temperature for different non-linear resistors according to the invention.

FIG. 3 is a diagrammatic representation of the current and temperature dependence of the electric resistance of a non-linear resistor in accordance with the invention, and

FIG. 4 shows the current and temperature dependence of the electrical resistance of a non-linear resistor in accordance with the invention at different re-oxidation temperatures.

The manufacture of non-linear resistors according to the present invention will first be described hereinafter.

1. Manufacture of the ceramic sintered bodies

SrCO.sub.3 and TiO.sub.2 were used as starting materials for the ceramic sintered body and La.sub.2 O.sub.3, Nb.sub.2 O.sub.5 or WO.sub.3 were used as doping metal oxides. In the preparation of the ceramic mass according to the compositions (Sr.sub.1-x La.sub.x)TiO.sub.3.yTiO.sub.2, Sr(Ti.sub.1-x Nb.sub.x)O.sub.3.yTiO.sub.2 or Sr(Ti.sub.1-x W.sub.x)O.sub.3.yTiO.sub.2 with 0.0005&lt;x&lt;solubility limit in the Perowskite phase and y=0.001 to 0.02, the TiO.sub.2 -excess with 0.001 to 0.02 has hence been chosen that there is always a small excess of Ti.sup.4+ -ions. As a result of this, a liquid sintering phase with the SrTiO.sub.3 is formed at a sintering temperature above 1400.degree. C.--it is assumed that it deals with the eutectic SrTiO.sub.3 --TiO.sub.2 occurring at .apprxeq.1440.degree. C., which may also occur at lower temperatures by the addition of dopants. A liquid sintering phase of this type favours the desired coarse granular grain growth.

For the manufacture of the ceramic bodies the following quantities were weighed-in.

For Sr(Ti.sub.0.99 Nb.sub.0.01)O.sub.3.0.01TiO.sub.2 : 59.056 g of SrCO.sub.3 31.96 g of TiO.sub.2 and 0.5316 g of Nb.sub.2 O.sub.5 ; for (Sr.sub.0.99 La.sub.0.01)TiO.sub.3.0.01TiO.sub.2 : 58.465 g of SrCO.sub.3 32.28 g of TiO.sub.2 0.6517 g of La.sub.2 O.sub.3 ; for Sr(Ti.sub.0.996 W.sub.0.004)O.sub.3.0.01TiO.sub.2 : 59.056 g of SrCO.sub.3 32.152 g of TiO.sub.2 0.371 g of WO.sub.3. They were mixed wet in a ball mill, for example, of agate. Pre-sintering at 1150.degree. C. for 15 hours is then carried out. The pre-sintered powder is again ground while wet (1 hour in a ball mill, for example, of agate). The ground material is then dried and the powders thus obtained are then granulated by means of a suitable binder, for example, a 10% aqueous polyvinyl alcohol solution. The granulate is then compressed, for example, into discs having a diameter of .apprxeq.6 mm and a thickness .apprxeq.0.50 mm with a green density (density after compression) of approximately 55 to 60% of the theoretical density. Sintering of the pressed product is then carried out at a temperature of 1460.degree. C. for 4 hours in a reducing atmosphere. The atmosphere may consist, for example, of water vapour-saturated mixed gas consisting of 90% by volume of N.sub.2 and 10% by volume of H.sub.2. Since the oxygen partial pressure of the mixed gas is determined by the ratio of the two partial pressures P.sub.H.sbsb.2 /P.sub.H.sbsb.2.sub.O, the mixed gas should be saturated with H.sub.2 O at .apprxeq.25.degree. C. so as to create a standard reducing atmosphere. During the sintering it should be ensured that coarse granular structures occur preferably at sintering temperatures above 1440.degree. C.

The reducing sintering is to be carried out in a closed furnace, for example, a tubular furnace is suitable. Excess reducing gas should preferably flow away through a bubble counter so as to create a stable sintering atmosphere.

Sintered bodies manufactured in this manner are semiconductive and show no open porosity anymore.

By re-oxidation of these sintered bodies in an oxidizing atmosphere, for example in air, electrically highly insulating oxide layers as grain boundary layers are produced in the semiconductor grain structures of the sintered bodies. Experiments which led to the present invention were carried out in different conditions:

(a) at a fixed re-oxidation duration of 120 minutes at different temperatures of 900.degree. C., 1000.degree. C., 1100.degree. C., 1200.degree. C., or 1300.degree. C.

(b) at a fixed temperature of 1100.degree. C. and different re-oxidation durations of 5 minutes, 15 minutes, 30 minutes, 60 minutes, 120 minutes or 240 minutes.

2. Manufacture of non-linear resistors

Electrodes of suitable metals, preferably of gold, were provided, for example by vapour deposition, on sintered bodies prepared as described above so as to form a resistor. For better adhesion of the electrode metal it is recommendable first to provide on the ceramic body a suitable adherent layer as an intermediate layer between ceramic and electrode metal; for example, a Cr-Ni-layer is suitable.

Notes for special compositions:

(Sr.sub.1-x La.sub.x)TiO.sub.3.yTiO.sub.2 (0.0005&lt;x&lt;solubility limit of the La in the Perowskite phase; y=0.001 to 0.02): when x&lt;0.0005, the bodies to be sintered oxidize too rapidly, and the reproducibility of the results is no longer ensured.

The upper limit of x appears from the solubility limit of the La in the Perowskite phase. Optimum results were achieved with sintered bodies having a grain structure with grains of a diameter of 80 to 120 .mu.m with x=0.01 and y=0.01 at a sintering temperature of 1460.degree. C. in a reducing atmosphere.

Sr(Ti.sub.1-x Nb.sub.x)O.sub.3.yTiO.sub.2 (0.005&lt;x&lt;solubility limit of the Nb in the Perowskite phase; y=0.001 to 0.02): the same as described above for the La-dopings also applies to the lower limit of x; from x.apprxeq.0.03 and more, homogeneous microstructures were no longer obtained in a reproducible manner. Optimum results were achieved with sintered bodies having a grain structure with grains of a diameter of 60 to 80 .mu.m with x=0.01 and y=0.01 at a sintering temperature of 1460.degree. C. in a reducing atmosphere. Sr(Ti.sub.1-x W.sub.x)O.sub.3.yTiO.sub.2 (0.0005&lt;x&lt;solubility limit of the W in the Perowskite phase; y=0.001 to 0.02): the same as described above for the La-dopings also applies to the lower limit of x; from x.apprxeq.0.01 predominantly fine-granular microstructures were observed, from x.apprxeq.0.06 and more a separation of foreign phases occurs increasingly in the microstructure which consists of SrWO.sub.4 and TiO.sub.2. Optimum results were achieved with sintered bodies having a grain structure with grains of a diameter of 60 to 80 .mu.m, with x=0.004 and y=0.01 at a sintering temperature of 1460.degree. C. in a reducing atmosphere.

3. Results

FIG. 1 shows the current-voltage characteristic of a resistor having VDR-characteristic with a sintered body of the composition (Sr.sub.O.sbsb.2.sub.99 La.sub.0.01)TiO.sub.3.0.01TiO.sub.2. The current density in mA/cm.sup.2 is plotted against the applied electrical field strength in kV/cm. In order to adjust the VDR-characteristic, the sintered body was re-oxidized in an oxidizing atmosphere at 1300.degree. C. for 2 hours after having been sintered in a reducing atmosphere, as already described. The sintered body had a cross-sectional area of approximately 0.19 cm.sup.2 and a thickness of approximately 400 .mu.m.

FIGS. 2a to 2c show the operational voltages at a current of 1 mA and the current indices .beta. for resistors having sintered bodies of the following compositions dependent on the re-oxidation temperature:

FIG. 2a: Sr(Ti.sub.0.99 Nb.sub.0.01)O.sub.3.0.01TiO.sub.2 FIG. 2b: (Sr.sub.0.99 La.sub.0.01)TiO.sub.3.0.01TiO.sub.2 FIG. 2c: Sr(Ti.sub.0.9996 W.sub.0.004)O.sub.3.0.01TiO.sub.2.

The numbers beside the points in the individual curves indicate the temperatures at which re-oxidation was carried out in an oxidizing atmosphere for 2 hours. A .beta.=1 means a purely ohmic resistance with NTC-properties (in FIGS. 2a to 2c the operational voltage is plotted logarithmically so that values for .beta.=1 are not shown within the range of U-values given). Of all three material groups this value is reached prior to the re-oxidation and at low re-oxidation temperatures, respectively (see Tables 1 and 2). With the geometry chosen the resistance of the sintered body prior to the re-oxidation typically lies at values of a few ohms. This results in a resistivity of approximately 10 .OMEGA..cm. FIGS. 2a to 2c furthermore show that when the re-oxidation temperature is increased the .beta. value decreases very markedly, so a typical VDR-behaviour begins. In particular the sintered bodies doped with La show that already at low operational voltages of a few volts .beta. values of 0.16 were achieved. By further increasing the re-oxidation temperature, the operational voltage can be varied in a wide range with .beta. approximately constant.

FIG. 3 shows diagrammatically general pattern of the current and temperature dependence of the electric resistance for non-linear resistors according to the invention. In (R) (the logarithm of the electric resistance R) is plotted against the reciprocal temperature 1/T. The range I characterizes a pure NTC-behaviour, the range II characterizes a VDR-behaviour.

A range of constant increase in such a diagram results in a relation of R and T according to R=A. e.sup.B/T

(R=resistance, measured in .OMEGA.; A=a constant having the dimension .OMEGA. essentially dependent on the outer shape of the resistor; e=base of the natural logarithm; B=a constant having the dimension K dependent on the outer shape and on the NTC-material; T=absolute temperature of the resistor in K). In wide temperature ranges such a linear relation is given. In principle, however, two ranges are to be distinguished. A range I in which a high value of B, that is a strong temperature dependence, is to be observed with a simultaneously absent dependency on the applied current (NTC-behaviour) and a range II in which InR exhibits a flat characteristic, that it has a small temperature dependence. In this case, however, a considerable influence of the current on the resistance is to be noted, so this is the range in which VDR-properties predominate. Both ranges merge into each other with each time different current values and temperatures.

FIG. 4 shows graphically the current and temperature dependence of the electric resistance of a non-linear resistor of the composition (Sr.sub.0.99 La.sub.0.01)TiO.sub.3.0.01 TiO.sub.2. The logarithm InR of the electric resistance (R) is plotted against the reciprocal absolute temperature. The sintered bodies were re-oxidized in air at temperatures of 1100.degree. C. and 1200.degree. C., respectively, for 2 hours after having been sintered in a reducing atmosphere as described above.

The influence of the higher re-oxidation temperature is experienced here in a shift of the set of curves to higher resistance values.

The following Table 1 gives the numerical values of the non-linearity factor .beta. and the operational voltage U.sub.1mA for individual dopants and re-oxidation temperatures.

TABLE 1__________________________________________________________________________Re-oxidation Sr(Ti.sub.0.99 Nb.sub.0.01)O.sub.3.0.01 TiO.sub.2 (Sr.sub.0.99 La.sub.0.01)TiO.sub.3.0.01 Sr(Ti.sub.0.996 W.sub.0.004)O.sub.3.0 .01 TiO.sub.2temperature (.degree.C.); Operational Operational current operational(Re-oxidation Current index voltage Current index voltage index voltageduration 2h) .beta. U.sub.1mA (V) .beta. U.sub.1mA (V) .beta. U.sub.1mA (V)__________________________________________________________________________ 900 0.75 0.09 1 0.002 0.44 0.51000 0.6 0.21 0.94 0.017 0.19 5.11100 0.57 0.47 0.34 0.5 0.19 7.31200 0.24 3.2 0.16 2.2 0.15 12.51300 0.13 20.6 0.17 6.6 0.13 16.8__________________________________________________________________________ Thickness of the sintered bodies uniformly .apprxeq. 400 .mu.m Diameter of the sintered bodies uniformly .apprxeq. 5 mm.

Table 2 shows the influence of the re-oxidation duration at a constant temperature of 1100.degree. C. In this case also it is confirmed that an increasing strength of the re-oxidation given by a higher temperature or longer re-oxidation duration, influences the values for the current index .beta. and the operational voltage U.sub.1mA (see also FIGS. 2a to 2c).

TABLE 2__________________________________________________________________________Re-oxidation Sr(Ti.sub.0.99 Nb.sub.0.01)O.sub.3.0.01 TiO.sub.2 (Sr.sub.0.99 La.sub.0.01)TiO.sub.3.0.01 TiO.sub.2 Sr(Ti.sub.0.996 W.sub.0.004)O.sub.3 .0.01 TiO.sub.2(min) Operational Operational current operational(re-oxidation Current index voltage current index voltage index voltagetemperature 1100.degree. C.) .beta. U.sub.1mA (V) .beta. U.sub.1mA (V) .beta. U.sub.1mA (V)__________________________________________________________________________ 5 0.68 0.13 1 0.003 0.27 1.4 15 0.59 0.22 1 0.005 0.21 2.5 30 0.64 0.23 1 0.010 0.22 3.0 60 0.57 0.36 0.75 0.07 0.15 5.7240 0.62 1.6 0.2 1.1 0.15 7.2__________________________________________________________________________ Thickness of the sintered bodies uniformly .apprxeq. 400 .mu.m Diameter of the sintered bodies uniformly .apprxeq. 5 mm.

FIG. 5 shows a cross-section through a nonlinear resistor with a ceramic sintered body (1) according to the invention. The body is on both sides provided with electrode-layers (2), and metal electrode caps 3, on which connection leads (4) have been secured.

Claims

1. A non-linear resistor having a ceramic sintered body consisting of a polycrystalline alkaline metal titanate doped to N-type conductance with a metal oxide and which sintered body is provided with electrodes on opposite located surfaces thereof characterized in that said body has a Perowskite structure, consists of an alkaline earth metal titanate containing excess TiO.sub.2, has a formula selected from the group consisting of (Sr.sub.1-x Ln.sub.x) TiO.sub.3.yTiO.sub.2 and Sr(Ti.sub.1-x Me.sub.x)O.sub.3.yTiO.sub.2 wherein Ln is a rare earth metal, Me is a metal having a valence of at least 5, 0.0005&lt;x&lt;solubility limit of the Ln in the Perowskite phase and y=0.001 to 0.02 and which sintered body is provided on its grain boundaries with insulating layers produced by re-oxidation.

2. A non-linear resistor as claimed in claim 1, characterized in that lanthanum is the rare earth metal.

3. A non-linear resistor as claimed in claim 1, characterized in that niobium is the metal of a valance of at least 5.

4. A non-linear resistor as claimed in claim 1, characterized in that tungsten is the metal of a valance of at least 5.