CH668591A5 - High dielectric internal grain boundary barrier layer ceramic - Google Patents

Abstract

A sintered layer is coated on semiconductive SrTiO3 grain, utilising sol. gel technique.

Description

       

  
 



   DESCRIPTION



   The present invention relates to ceramic compositions with a high dielectric constant comprising conductive particles or grains and an insulating barrier layer on the border of these grains; these compositions are suitable for the manufacture of high capacity ML capacitors. The production of multilayer capacitors makes it possible to increase the capacity and, simultaneously, to reduce the size of the capacitors; such structures are obtained by stacking the electrodes, two by two, alternately, the even and odd electrodes, respectively, being connected together by the edge so that the sum of the areas of the elements of each group constitutes the total effective area of the capacitor.



   The use of grain boundary barrier layer ceramics for the manufacture of high capacity miniaturized capacitors (capacitors
GBBL) has been known for many years. Thus, already in 1960, the document GB-A-849-938 mentioned the densification by sintering at 1450 C under hydrogen of grains of BaTiO3 of high purity so as to provide BaTiO3, a porous semiconductor having an average particle size of 1 llm . By oxidation with air under atmospheric pressure between 650 and 1000 "C, an intergranular insulating barrier layer of oxide with a thickness of about 10 nm was incorporated into this material.

  The properties of the capacitors obtained from such dielectrics were as follows: effective permittivity x (or s) approximately 100,000, tangent of the loss angle 6 approximately 0.03 (Q = approximately 50 to 5 kHz and 5 V rms rms ).



   This material being disadvantaged by excessive porosity, instability at temperature changes and a fairly low breakdown voltage, it was later abandoned in the art and replaced by semiconductor strontium titanate doped with transition metals such as lanthanum, niobium and tungsten, and sintered, either without additives, or mixed with materials capable of providing the barrier layer in the form of a liquid sintering phase, such materials comprising for example zirconia, bismuth tungstate, lead germanate and others (see for example JP-A-77.10595 (1977).

  When sintering is carried out without said liquid sintering phase, the limiting intergranular insulating material can then be introduced by coating the surface of the sintered plate with a metal oxide and by diffusing the latter in the molten state by baking in an atmosphere. oxidizing in the ceramic body (see JP-A-75-27995).



   Document JP-A-74-129896 also discloses the preparation of a sintered ceramic by molding a grout containing SrTiO3, Nb2Os, GeO2 and an organic binder in the form of discs and by baking the discs molded at 1350-1480 " C in an atmosphere containing hydrogen. The insulating intergranular limit phase is carried out by coating the disc with a Bi2O3 powder and by baking it at 1300 C, which causes the diffusion of the molten oxide in the ceramic. thus obtained materials having pennittivities of the order of 3-5 x 104 and Q factors of around 100.



   Document JP-A-76-648 discloses the sintering of mixtures comprising SrTiO3, Nb2Os and CoO2 at 1450 C in an N2 / H2 atmosphere. The limiting phase of coating of the grains comes from the diffusion in the liquid state at 1100 ° C. of oxides capable of forming glasses, these being chosen from SiO2, B203, GeO2, P2Os,
Al203, MgO, ZnO, Bi2O3 and TiO2.



   Document JP-A-83-97820 relates to the preparation of semiconductor ceramics by pressure molding and sintering at 1350 1450 C under a 1:99 atmosphere of hydrogen and nitrogen of mixtures of SrTiO3, WO3, Ge2O, Al203. To form the insulating barrier layer, a molten mixture of
PbO, B203 and Bi2O3. As optimal electrical properties, permittivity (e) and factor Q values of about 100,000 and 200 were obtained, respectively, using SiO2 and Al203 in a weight ratio of about 1.5 to 5.0. The mean grain size was around 60 to 120 µm and the resistivity of the interstitial boundary layer was around 2 x 109 Q cm.



   Document JP-A-77-10596 describes the doping of SrTiO3 by mixing with oxides such as Nb2Os, Ta2Os, WO3 or TiO2 and baking at around 14000 C under a reducing atmosphere. An amount of approximately 1% by weight of doping agent is sufficient to obtain resistivities of less than 10 cm.

  This reference also mentions the use of SiO2 as a sintering additive, this material favoring the magnification of semiconductor grains on sintering and of grains of a size greater than 10 μm, having preference for bilayer capacitors (BL). This reference also teaches that, by adding ZrO2 to the SrTiO2 to be sintered, the Q factor and the resistivity are increased. The limit phase was provided, in this case, by diffusion of a mixture in the molten state of PbO, B203 and Bi2O3.



  As typical values of the electrical properties, we cite e 30000 and Q 200.



   The above references generally describe relatively large grain ceramics which are suitable for the manufacture of two-layer capacitors. However, the effective permittivity (or effective dielectric constant) of barrier layer ceramics comprising insulating layers at the periphery of the semiconductor grains depends on the size of the grains and on the thickness of the intergranular insulating layer. The relation providing the capacitance of a capacitor according to the properties of the ceramic and its dimensions is as follows (see for example Ultra-High Dielectric
Capacitance in TiO2 Ceramics by M. F. Yan and W. W. Rhodes,
Bell Laboratories, Murray Hill, N.J. 07974, Adv. Ceram. (1983)
Addition Interface Electr.

  Ceram. [7], 226-238):
 C = Kgb x D / dx S / e, where S and e represent the area and the thickness in cm of the dielectric ceramic plate, respectively, and D, Kgb and d represent, respectively, the diameter of the grains, the intrinsic dielectric constant of the interstitial insulating boundary layer and its thickness, that is to say the intergranular distance. Thus, permittivity values of the order of 104 to 105 are obtained with grains of 10 to 50 μm and separation layers of 102 to 103 μm (0.1 to 1 μm).



   However, dielectrics with grains larger than about 5-10 µm are not suitable for ML capacitors with layer thicknesses of the order of 30 µm or less; also, efforts are made to simultaneously reduce the grain size and the thickness of the intergranular barrier layer. Interesting results have been obtained in this sense by sintering in the presence of a liquid phase consisting of lead germanate and borates (see Characterization of In ternal Boundary Layer Capacitors by H. D. Park and D. A.

  Payne,
Advances in Ceramics, Vol. 1, pp. 242-253, 1982), the grains used having approximately 0.5 to 2 μm in diameter and the thickness of the barrier layer being of the order of 100 nm (0.1 μm). We thus obtained effective permittivities in the 103 range with relatively little dielectric losses (tangent 8 around 10-2; Q = 100), and slight variations due to temperature (5-10%); see also documents US-A-4,158,219 and 4,237,084. The parameter called tangent 6 represents the dissipation factor, 8 being the difference (in angular units) between 90 "and the phase-shift angle O voltage-intensity of the alternating current applied to the capacitor.

 

  Ideally, if the capacitor shows no loss,
O would be 11/2 and 6 would be zero. Q is the inverse of tangent 8; Q = 1 / tangent 8.



   However, it was desirable to make further improvements to the results of the known technique and to provide ceramics of even higher permittivity without conceding too much loss with respect to thermal stability and the Q factor. Such ceramics are defined in claim 1 appended. These ceramics are produced by the process defined in claim 2. This process constitutes a step forward in the desired direction; this step was taken when the present inventors discovered with surprise that the liquid phase of sintering and densification can be provided by coating the semiconductor grains with a layer of agent promoting sintering (sintering aid) by the technique known as sol -gel.



  It is known that this technique consists in using, when it is necessary to have metal oxides of very high purity, organic compounds of said metals and to convert these subsequently into glasses or ceramics by, successively, hydrolysis, polycondensation and densification thermal. In the present case, the main advantage of using this technique results in improved control of the thickness of the barrier layer which can be exerted by leaving the grains of the conductive powder for a longer or shorter period of time. semiconductor) in contact with a solution of organometallic compounds, so that the insulating barrier layer is formed.

  In other words, the quantity of coating material which is deposited in the wet state can be varied by adjusting the operating conditions, which results in a corresponding variation in the electrical properties of the ceramic resulting from the process. Thus, the effective thickness of the interstitial layer coating the grains can be as low as 0.8 to 1.5 nm although, naturally, it can be made thicker by extending the contact time with the impregnation solution, the temperature and the other reaction parameters (concentrations of dissolved materials, types of solvents, pH so as to act on the gelling rate, etc.). It is also possible to act on the thickness of the interstitial layer by repeating the coating operation after drying of the powder following a first coating.

  The effective thickness of the coating obtained as described above and in claim 1 can be estimated by comparison with a control plate, for example made of glass, which is subjected to identical operating conditions and which is then subjected (i.e. after drying and heating) to optical measurements. In general, advantageous results are obtained when the weight gain undergone by the powder following the coating treatment of the semiconductor grains is of the order of 10-4 to 10-2 or more (i.e. 0.01 to 1% or more).



  Another advantage of the present invention is to allow a reduction in the sintering temperature, which results in a reduction in the tendency of the grains to enlarge during sintering.



   In general, the doped powder is impregnated with
SrTiO3 by mixing it (the grains of which have a size of about 0.8 to 5 μm) under an inert atmosphere (for example pure nitrogen or argon) with an organic solution of the compounds to be deposited to form the barrier layer. Solvents such as ethanol, propanol, butanol, acetylacetone and other water-compatible solvents are suitable; the concentration of organometallic compounds in the solution is approximately 5 to 20% by weight, but these limits are not critical and can be exceeded on both sides. Usually, a gelling catalyst can be incorporated into the solution, for example, small amounts of HCl or other mineral acid; bases, such as amines or NH4OH, are also suitable.

  These catalysts cause the formation of a gel layer on the surface of the grains, this gel layer being, after the fact, dried and heated in air to transform it into insulating metal compounds or into insulating oxides corresponding to the metals included in this layer of gel. As heat treatments, one can, for example, heat to 500-600 C, in air, for 1 to 2 hours or longer if desired, so that the layer densifies and the remaining organic residues are eliminated by oxidation. .



   As metal compounds which can be used to form the solutions of alkoxides, mention may be made of those of the metals generally used in the art to constitute the phase promoting sintering. For example, solutions containing lead hexanoate and a germanium alkoxide, or alkoxides of boron and bismuth, or alkoxides of silicon and aluminum may be suitable. By impregnation with such solutions, followed by drying and heating as indicated above, layers of lead germanate, lead borobismuthate or alumina silicate are obtained, respectively. Several successive layers in combination can sometimes be advantageous; in particular, one or more layers of alumina silicate can be supplemented with one or more layers of lead germanate.

  It is interesting to note that the presence of an aluminosilicate layer on the doped strontium titanate grains will have the effect of inhibiting the magnification of these semiconductor grains during sintering, which is advantageous and unexpected since a of the aforementioned references says precisely the opposite.



   It may be desirable, with regard to the implementation of the sintering step of the present method, to supplement the quantity of liquid phase provided by the abovementioned technique with an additional quantity of agent favoring sintering in the form of powder. To this end, the SrTiO3 powder provided with a coating layer is mixed with an additional quantity of sintering agent in dry pulverulent form and the mixture is then molded into the desired form using a binder. subsequently decomposable (as a binder, it is possible to use a volatile or easily combustible polymer, such as polyvinyl alcohol or polyvinyl butyral).

  As compounds which can be used as agents complementary to the liquid sintering phase to be incorporated in pulverulent form, mention may be made of compounds and oxides, such as PbO, GeO2, Bi203, H3BO3, BaCO3, P205 and SrTiO3. The use of SrTiO3 as a component of the liquid sintering phase during the sintering of the semiconductor doped SrTiO3 grains is new and provides notable advantages with regard to the permittivity and Q factor of sintered ceramic.

  The preferred additional sintering aid compositions are as follows (in% by weight):
PbO 78% - GeO2 22%; PbO 45% - B203 5% - Bizou 50%;
PbO 25% - B203 20% - Bizou 20% - GeO2 5% - BaO 10% P2O5 20%.



   When SrTiO3 is used as additional sintering aid for the liquid phase, it is preferably added at the rate of 5% by weight to one of the preferred compositions above.



   The doped semiconductor powder, for example niobium, used as internal granular phase in the present process can be obtained by mixing SrTiO3 and Nb2Os and baking the mixture under a reducing atmosphere, for example at 1450-1500 "C under a mixture 1-10% H2 / Ar or H2 / N2. Alternatively, a mixture of TiO2, Nb2Os and SrCO3 can also be subjected to such cooking.



   We can also use a different technique to prepare the
SrTiO3 doped with a metal, for example by impregnating a powder of SrTiO3 with an organic solution (of sol-gel type) of a metallic alkoxide and by proceeding in a manner quite similar to that described above, where it is supplying by the sol-gel technique the material constituting the limiting insulating phase of the semiconductor grains and simultaneously the sintering aid agent. Thus, the undoped SrTiO3 powder can be immersed in organic solutions containing a niobium alkoxide, wrung out, dried at 90 ° C. and heated in air to around 500-600 ° C. at the rate of a few ° C. / min (for example 4 C / min are suitable).

  Then a cooking-reduction operation is carried out, for example it is heated 2-3 h at 1450-1500 "C in a reducing atmosphere, in particular in a mixture 1-10% (V / V) H2 / Ar, H2 / N2 or CO and CO2. Then, to obtain a doped powder of suitable particle size, the reduced material is powdered and ground in a ball mill until grains of the desired size are obtained, for example 0.5-2 llm on average.



   Alternatively, the above-mentioned sol-gel doping treatment can be applied to TiO2 powder and, after doping and drying, this powder is mixed with an equivalent of strontium carbonate as indicated above, the resulting mixture being then reduced to hot as described and ground to the desired fineness.



   According to another variant, an organic solution is used containing titanium and niobium alkoxides and the formation of a gelled suspension of microparticles is caused by the addition, to this stirring solution, of a catalyst such as ammonia aqueous. This suspension is separated and dried in the form of TiO2 powder doped with Nb, it is mixed in desired proportions with
SrCO3 powder and a reduction is carried out at 1300-1500 C under an atmosphere containing hydrogen, as described above.



   The sintering conditions allowing the conversion of the grains of
SrTiO3 semiconductors mixed with the material incorporated by the sol-gel technique and constituting the liquid phase of sintering and, subsequently, coating of said grains, with or without additional supply of pulverulent material, so as to produce a granular ceramic with interstitial layer of high dielectric constant limit first of all compacting or molding the powder under pressure in the presence of a binder mixed with the powder so as to produce an object in green (a disc, a wafer, etc.),

   then this object is sintered and densified at a temperature usually between 1000 and 1300 C under an inert gas such as argon or nitrogen or in a reducing atmosphere so as not to risk losing the semiconductor properties of the doped Sorti03. As the reducing atmosphere, dilutions of hydrogen can be used in an inert gas such as nitrogen or a rare gas. Permittivities of the order of 106 were thus obtained accompanied by low Q factors. By an appropriate subsequent heat treatment, for example by air annealing, the Q factor and the mechanical strength of the ceramic can be raised, at the expense, to some extent, of the permittivity.



   In general, sintered discs were prepared by mixing the powder coated with the liquefiable phase (in the presence or absence of an additional quantity of powder) with 1-3% of a binder such as polyvinyl butyral (taken from 1-2 g of powder), by pressing the shaped sample by means of a piston press, the pressed discs having approximately 10-20 mm in diameter and 1-3 mm in thickness, by performing sintering 1- 6 h at a given temperature, by fixing electrodes on either side of the disc and by measuring its dielectric properties (capacitance and loss factor Q) by the usual means.

  The formula used is as follows (C measured in picofarads = 10-12 F):
 C = 0.0885 E S / e, from which we calculated (the permittivity, also designated by x), S being in cm2 and e in cm. The electrodes (Cu, Ag, Pt or Au) were applied by spraying under vacuum by known techniques or by screen printing using a paint or a metallic paste whose binder is destroyed during cooking; such an application can be done directly on the green object (cooking then corresponds to sintering) or afterwards on the object already sintered.



   It was also possible to improve the dielectric properties of the densified ceramic by a diffusion post-treatment similar to that known in the state of the art. For example, mixtures of various vitrifiable metal oxides were dispersed in binders and the pastes thus obtained were applied to the faces of the sintered ceramic discs; then it was heated in a range of temperatures of the order of 800 to 1000 C, so that the oxides melt and the liquid diffuses into the pores of the substrate.

  In some cases, it was possible to combine this treatment with the placement of the electrodes; thus, the pulverulent vitrifiable oxides were kneaded with a paste or a metallic paint (for example Ag or Pt), this paste was applied to the faces of the ceramic discs and the whole was cooked as already described above . During this firing, the molten oxides diffuse in the ceramic while the metal is deposited on the surface of this, thus providing the desired electrodes.



   Some of the preferred compositions for carrying out the above-mentioned diffusion treatment are given below: (a) Bizou 36% - CuO 4% - Ag paste 60% (b) PbO 32% - GeO2 8% - Ag paste 60% ( c) PbO 30% - GeO2 8% - SrTiO3 2% - Ag paste 60% (d) PbO 18% - Bizou 20% - B203 2% - Ag paste 60% (e) PbO 78.05% - GeO2 21.95 % (f) PbO 76% - GeO2 19% - BaO 5% (gl) PbO (g2) PbO 60% - PbF2 40% (h) PbO 20% - Bi203 20% - B203 40% - CuO 20% (i) PbO 66% - GeO2 12% - Bizou 20% - B203 2%
 According to a variant, we used,

   for diffusion, the vapors of said molten oxides. For the implementation of this variant, the solids were melted in a crucible (Pt) and the ceramic disks were placed in the opening thereof, fixing them by hooks at about 1- 3 cm from the level of the molten material.



  In this variant, use was preferably made of the composition (h) above (fusion 800 C) and a mixture (by weight) 4: 6 B203 / PbO (temperature of the molten liquid 900 C).



   The following examples illustrate the invention in detail.



  Example 1:
 A semiconductor doped SrTiO3 powder (I) was prepared by mixing 79.63 g of very pure SrCO3, 43.1 g of TiO3 (grain size 0.75 llm) and 1.01 g of Nb203 (0, 7 mole%). The mixture was heated 2 h at 1100 C in air (platinum crucible) and, after cooling, it was further heated 4 h at 1480 C in an H3 / Ar mixture at 10% H2 (V / V ) so as to ensure a reducing effect and to cause the appearance of the semiconductor properties. The powder was then ground for 24 h in a 250 ml plastic mill containing 20 WC / Co balls 9 mm in diameter and, as grinding liquid, a 3: 1 mixture (V / V) of petroleum ether and butanol.

  There was thus obtained a powder with a particle size of about 0.5-5. The powder was then wrung out and dried. Its resistivity, measured by the usual means, was 4.88 Rcm. The semiconductor powder was then immersed in an excess of an organic solution (PEG) containing 0.53 g of germanium tetraethoxide, 1.37 g of lead 2-ethylhexano'1.6 g of acetylacetone , 13 g ethanol and 10 mg HCl (in the form of a 0.1 N ethanolic solution).



  After standing for about 30 min, the powder was filtered, dried for 16 h at 90 ° C., then it was heated for 1 hour in air at 550 ° C. (4 ° C./min). This treatment was repeated four times in a row. Then, after incorporating 1-3% organic binder into the powder, the whole was pressed in the form of discs at 2.5 T / cm 2 as described previously (15 mm in diameter; 1 mm in thickness). These discs were sintered for 100 min at the conditions summarized in Table I below (the term flash indicates that the samples were quickly brought to the sintering temperature).



   Table I
EMI4.1


 <tb> <SEP> Sintering <SEP> Properties
 <tb> Sample <SEP> Heating
 <tb> <SEP> Atm. <SEP> <SEP> a <SEP> Q <SEP> R <SEP> (MQ) <SEP>
 <tb> <SEP> C / min <SEP> C <SEP>
 <tb> <SEP> I-A <SEP> air <SEP> flash <SEP> 1120 <SEP> 1140 <SEP> 5.5 <SEP> 2000
 <tb> <SEP> I-B <SEP> N3 <SEP> flash <SEP> 1120 <SEP> 2349 <SEP> 7.6 <SEP> 400
 <tb> <SEP> I-C <SEP> N2 <SEP> 7 <SEP> 1120 <SEP> 1606 <SEP> 20 <SEP>
 <tb> <SEP> I-D <SEP> N3 <SEP> flash <SEP> 1200 <SEP> 5267 <SEP> 11 <SEP> 120
 <tb> <SEP> I-E <SEP> N3 <SEP> 7 <SEP> 1200 <SEP> 3076 <SEP> 13 <SEP> 330
 <tb>
 After sintering,

   Pt electrodes were applied to the disks by sputtering in the usual manner and the properties of the dielectric were measured at 1.5 kHz using a Hewlett-Packard HP-4192-AIF electrotest apparatus high impedance lamp ohmmeter. The results of the measurements, in terms of permittivity e, factor Q and ohmic resistance are also shown in Table I.



  Example 2:
 A semiconductor SrTiO3 powder (I ′) was prepared as in Example 1, with the following difference: the reduction step was carried out for 5 h at 1480 C under a 10% (V / V) mixture d hydrogen and nitrogen. This powder was then ground (as indicated in Example 1) for 48 hours in the presence of 50 tungsten carbide balls. After removal of the grinding solvent by drying, four successive coating treatments were applied to this powder using the lead and germanium solution described in the previous example, namely immersion in the liquid, drying for 2 hours at 95 ° C. , heating (flash) to 550 "C and holding 1 hour at this temperature in air.

  Then 1.5% by weight of polyvinyl butyral (PVB) was mixed with this powder using a solution of this polymer in a 75:25 (V / V) mixture of petroleum ether / t-butanol, it was dried, passed through a 300 m mesh screen and molded under pressure in a green disc of I g (3 t / cm3). Was heated in air for 1 hour at 5500 C to destroy the binder (0.5 C / min).



   The green discs were then sintered under the conditions indicated in Table II and the dielectric properties were measured as described in Example 1. The results are collated in the same Table II.



   Table II
EMI5.1


 <tb> <SEP> Sintering <SEP> properties
 <tb> <SEP> dielectrics <SEP>
 <tb> Sample <SEP> Sintering <SEP> dielectrics <SEP>
 <tb> <SEP> Atm. <SEP> Heated <SEP> emp. <SEP> Time <SEP> ± <SEP> <SEP> Q <SEP> R <SEP> (I Q) <SEP>
 <tb> <SEP> (0Cjmin) <SEP> (0C) <SEP> I <SEP> (min) <SEP>
 <tb> <SEP> I-A <SEP> N2 <SEP> 7 <SEP> 1250 <SEP> 120 <SEP> 6873 <SEP> 2.1 <SEP> 75
 <tb> <SEP> I'-B <SEP> N2 <SEP> 10 <SEP> 1350 <SEP> 120 <SEP> 9279 <SEP> 2.2 <SEP> 17
 <tb> <SEP> r-c <SEP> N2 <SEP> <SEP> 10 <SEP> 1250 <SEP>

   180 <SEP> 6692 <SEP> 2.8 <SEP> 10
 <tb> <SEP> r-D <SEP> N3 + air <SEP> 10 <SEP> 1250 <SEP> 180 <SEP> 478 <SEP> 21 <SEP> 5000
 <tb> <SEP> r-E <SEP> N3 <SEP> 10 <SEP> 1100 <SEP> 240 <SEP> 840000 <SEP> 0.6 <SEP> 2.4
 <tb> <SEP> r-F <SEP> N3 <SEP> 7 <SEP> 1350 <SEP> 60 <SEP> 240000 <SEP> 0.4 <SEP> 22
 <tb> <SEP> I-G <SEP> N3 <SEP> 8 <SEP> 1060 <SEP> 120 <SEP> 4857 <SEP> 2.9 <SEP> 65
 <tb>
 E was air annealed for varying lengths of time at various temperatures. The results, recorded in Table III, indicate that this post-treatment somewhat improves the Q factor.



   Table III
EMI5.2


 <tb> Temperature <SEP> Time <SEP> x <SEP> 105 <SEP>
 <tb> <SEP> ('C) <SEP> (h)
 <tb> <SEP> 650 <SEP> 1 <SEP> 3.6 <SEP> 1,2
 <tb> <SEP> 780 <SEP> 1 <SEP> 1.6 <SEP> 2.4
 <tb> <SEP> 800 <SEP> 3 <SEP> 1,2 <SEP> 2.6
 <tb>
 By electromicrography, it was found that the particle size of this sample was approximately 2 μm. The insulating barrier layer e was about 1 to 100 nm.



  Example 3:
 A solution of niobium ethoxide (S2) was prepared by mixing 10 g of acetylacetone, 4.78 g of niobium ethoxide, 24 g of n-butanol and 1.2 mg of HCl (supplied in the form of '' 0.1 N ethanolic solution).



   About 15 g of very pure strontium titanate powder (Kyorix 1150 / A) was impregnated with this solution at room temperature and then, after gelation, the product was drained and heated for 2 h at 600 "C (at speed of 2 "C / min). This doping treatment was repeated, after which the powder was subjected to a reduction for 2 h at 1490 C (temperature rise rate 7 "C / min) under a 10% (V / V) mixture of hydrogen and d Argon A semi-conductive powder (STK-III) was obtained which was ground as in the previous examples; its resistivity, measured by the usual means, was 1.43 ohmcm.



   The grains of this powder were coated by soaking in a solution (PG2) containing 12 g of acetylacetone, 1.06 g of Ge (OEt) 4, 2.74 g of Pb ethyl-2-hexanoate, 23.32 g of n-butanol and 268.5 ml of 0.1N ethanolic HCl solution (0.98 g of HCl).



  After impregnation and gelation, the powder was drained, dried and then heated in air for 2 h at 500 C (temperature rise, 5-7 "C / min). This coating operation was repeated 2 times (sample C4-W) and 4 times (samples C4-L to V) Then these powders were pressed in the form of discs as in the previous examples and they were sintered as summarized in Table IV below. , which also indicates the dielectric properties of sintered discs.



  Comparative example 3a:
 A semiconductor powder of SrTiO3 (STK-I) was prepared as in Example 3, but using for doping a solution (SI) containing 5 g of acetylacetone, 2.39 g of niobium ethoxide, 12.1 g of ethanol and 0.6 g of HCl (0.1 N ethanolic solution). After the impregnation carried out as in Example 3, the reduction was carried out at 1483 C under 10% (V / V) of H2 / Ar. The resistivity of this powder was practically identical to that of the STK-III powder above.



   The insulating barrier phase was provided as in the prior art by grinding the doped SrTiO2 powder with a certain amount of powder (PGO) obtained by intimately mixing (parts by weight) 19.91 g of PbO and 5.49 g of GeO2. In this case, the proportion by weight of the PGO powder relative to the mixture with doped SrTiO3 was 5%.

  The mixture of oxides was melted at 900 "C in air, after which the block was reduced to powder and ground, as explained before, in a plastic ball mill in the presence of WC and a 3: 1 (V / V) mixture of petroleum ether and t-butanol, to a particle size of approximately 0.5-5 μm. The sintering in the form of test discs was carried out as a summary in Table IV, in which the electrical properties are reported for comparison.



   Table IV
EMI5.3


 <tb> <SEP> Sintering <SEP> Properties
 <tb> <SEP> dielectrics
 <tb> <SEP> Samples
 <tb> <SEP> Atm. <SEP> Heating <SEP> remp, <SEP> Time <SEP> E <SEP> Q
 <tb> <SEP> (C / min) <SEP> (C) <SEP> (min) ¯
 <tb> <SEP> C4-L <SEP> N2 <SEP> 7 <SEP> 1100 <SEP> 60 <SEP> 3522 <SEP>
 <tb> <SEP> M <SEP> N3 <SEP> flash <SEP> 1114 <SEP> 33 <SEP> 1788 <SEP>
 <tb> Test <SEP> C4T <SEP> N2 <SEP> 7 <SEP> 1250 <SEP> 120 <SEP> 10300 <SEP>
 <tb> Ex.

   <SEP> 3 <SEP> C4-U <SEP> N3 <SEP> 7 <SEP> 1000 <SEP> 270
 <tb> <SEP> 1060 <SEP> 120 <SEP> 3374 <SEP> 14.3
 <tb> <SEP> C4V <SEP> N2 <SEP> 6 <SEP> 1250 <SEP> 120 <SEP> 5762 <SEP> 3.9
 <tb> <SEP> C4W <SEP> N2 <SEP> 6 <SEP> 1250 <SEP> 120 <SEP> 7155 <SEP> 4.8
 <tb> ControlC3-K <SEP> N2 <SEP> 7 <SEP> 1100 <SEP> 60 <SEP> 2839 <SEP>
 <tb> Ex.

   <SEP> 3a <SEP> C3-N <SEP> N2 <SEP> flash <SEP> 1114 <SEP> 33 <SEP> 1341 <SEP>
 <tb> <SEP> C3-S <SEP> N2 <SEP> 7 <SEP> 1250 <SEP> 120 <SEP> 7216 <SEP> 2.4
 <tb>
 These results indicate that the method of the invention provides samples whose dielectric properties are superior to those of the control samples and that, on the other hand, the application of three layers of material intended to provide the intergranular barrier gives superior results. to the application of five coats.



  Example 5:
 A first solution was prepared (A) by mixing the following ingredients: 28.55 g of Ti (OEt) 4, 100 g of alcohol and 0.56 g of Nb (OEt) s, then diluted with alcohol to make 626 ml at room temperature. To this solution was added a solution (B) containing 13.51 g of 0.5 N aqueous solution of NH40H and alcohol to make 626 ml.



   The mixture resulting from the addition of B to A had the following titers: 0.1 M titanium and 0.59 M water; it also contained 0.054 mole of ammonia. The mixture gave a fine precipitate and, after 1 hour with stirring, 830 ml of water was added and stirring was continued for 15 min. Then after having left to stand for 16 h, the clear liquid was siphoned off and replaced with 1 l of alcohol. The precipitate was separated by filtration and redispersed for 1 hour in 11% boiling alcohol at reflux. The precipitate was re-filtered and dried overnight at 900 C. The cake was reduced to a fine powder, it was heated for 1 hour in air at 5500 C (1 C / min).



   TiO2 doped with niobium (8.63 g) was mixed with 15.59 g of
SrCO3 and the whole heated for 3 h in air at 1300 C. The resulting strontium titanate contains 0.7 mol% of niobium. We then proceeded to its reduction in order to obtain a semiconductor product, we operated as already described by 4 hours of heating at 1480 "C under a hydrogen / nitrogen mixture at 10% (V / V). Then we finally ground the powder in a plastic ball mill containing 50 balls of
WC and using as organic carrier phase a 3: 1 mixture (V / V) of petroleum ether and t-butanol, as already described previously. After air exchange for 4 hours at 90 "C, the powder was finally heated for 1 hour at 5500 C (4 C / min).

  The resistivity was approximately the same as that of powder I of Example 1.



   The coating intended to provide the insulating intergranular phase was applied as described in Example 1 using the solution defined as PGI (see Example 6). Four successive layers were applied with, between them, a drying operation of 4 hours at 90 "C; then it was gradually heated (4 C / min) to 550" C, temperature maintained for 1 hour thereafter, which provided a coated powder (V).

  This powder was pressure molded into discs as described above and these discs were sintered under the conditions given in Table V; electrodes were applied to the discs according to the usual method and the dielectric properties of the discs were measured as in the previous examples.



   Table V
EMI6.1


 <tb> <SEP> Properties
 <tb> <SEP> Sintering <SEP> dielectrics
 <tb> Samples <SEP>
 <tb> <SEP> Atm. <SEP> Heating <SEP> Temp. <SEP> Time <SEP> E
 <tb> <SEP> ("C / min) <SEP> (C) <SEP> (min)
 <tb> <SEP> V-A <SEP> N2 <SEP> 7 <SEP> 1200 <SEP> 100 <SEP> 3453 <SEP> 8.4
 <tb> <SEP> V-B * <SEP> N2 <SEP> 7 <SEP> 1200 <SEP> 100
 <tb> <SEP> N2 <SEP> 8 <SEP> 1350 <SEP> 60 <SEP> 9950 <SEP> 2.3
 <tb> <SEP> V-C <SEP> N2 <SEP> 7 <SEP> 1250 <SEP> 120 <SEP> 4490 <SEP> 3.5
 <tb> * The V-B sample was heated during the first period to
 1200 "C, then during the second, 1 h at 1350" C.



   The ceramics produced according to Examples 1 to 5 have been found to be applicable to miniaturized multilayer capacitors of high capacity.



   According to a modification, the niobium doped SrTiO3 semiconductor powder can be obtained as follows: TiO2 is first doped by dipping it in a niobium alkoxide solution, this operation being followed by the same treatment, described more high, used to boost SrTiO3 by sol-gel (see example 3). Then the TiOB treated with niobium is mixed with the required amount (1 equivalent) of a strontium salt, generally
SrCO3, and the mixture is reduced to 1300-1500 "C in a reducing atmosphere containing hydrogen.



  Example 6:
 The following ingredients were mixed and ground in a zirconia mill in the presence of 1 kg of ZrO2 beads: 923.78 g (6.26 moles) of SrCO3, 500 g (6.26 moles) of TiO3 and 11.72 g (0.044 mole) of Nb2O5. As the carrier phase, water was used. The powder cake was air dried, then heated for 2 hours at 1100 ° C. in air in an alumina crucible (temperature setting 400 "C / h). We then proceeded to a reduction by heating 2-4 h at 1450-1480 "C under a 10/90 (V / V) mixture of hydrogen / nitrogen. The heating rate being approximately 300-420 "C / h. The reduced product was then ground in a polyethylene mill in the presence of WC / Co beads to a particle size of 0.5 to 5 .mu.m.



  As the grinding liquid, a 75/25 mixture of petroleum ether / t-butanol was used.



   The ground doped SrTiO3 thus obtained was washed with acetone and, after having dried it, it was brought into contact for approximately 30 min, in a glove box, with a chosen solution of the sol-gel type. For this, we used the following solutions, called HM9SIA and
ERP.



   HM9SIA (abbreviated HM): Was mixed and heated 30 min at 70 "C 41.6 g of Si (EtO) 4.30 g of n-butanol, 7.9 g of di-sec-butoxydeacetylacetate aluminum chelate and 1.84 g of HCl (in the form of a 0.1 N solution in ethanol) After cooling, 3.3 g of H2O and 182 g of EtOH were added. at 500 C and allowed to cool.



   PGI: A mixture of 15 g of acetylacetone, 2.12 g of germanium tetraethoxide, 5.48 g of Pb 2-ethylhexanoate, 25.5 g of n-butanol and 1 was brought to boiling for 2 hours at reflux. 76 g HCl (as 0.1 N solution in ethanol); allowed to cool and used at room temperature.



   After bringing the ground reduced SrTiO3 into contact with the alkoxides solution for a sufficient time under nitrogen, the grout was drained on sintered glass under reduced pressure, then it was dried in the air for 16 h at 90 "C. The powder was then densified by heating it for 1 h at 550 "C, the rate of temperature rise being 60 to 240" C / h depending on the case. During this treatment, all the organic residues are destroyed and eliminated; if necessary , this treatment can be carried out in oxygen to accelerate the combustion processes. The densified powder is now ready to be molded and sintered, either without the addition of other additives, or after mixing with an additional quantity of liquefiable powder phase. .



   To obtain the pressed discs, the titanate powder was mixed with about 1.5% by weight of polyvinyl butyral (PVB) in an acetone / isopropanol solution. To do this, the required amount of PVB is added to the dispersion of SrTiO3 in the form of a 20 g solution of PVB per 120 g of a 1: 1 mixture of acetone / n-butanol.



   After having stirred for 30 min, it is evaporated, the powder coated with PVB is passed over a 300 μm mesh screen and it is pressed in the form of discs in a stainless steel mold at 2-5 t / cm 2.



  Then the green disks are heated for 1 hour in air at 550 "C to destroy the binder (0.5-1" C / min), after which the sintering is carried out.



   This sintering was carried out in Al203 tubular furnaces equipped with pipes for the introduction of gas, that is to say on one side of pure nitrogen, on the other a 10/90 mixture (V / V) hydrogen / nitrogen. The flow of each of these gases was controlled by valves and flow meters. By appropriate adjustment of these members, it was possible to precisely regulate the hydrogen content of the gas in which the sintering is carried out, and this within an approximate range of 0.01 to 5% by volume. H2 / N2 ratios in the range of 0.1 to 1% have been found to be advantageous. The sintering temperatures vary between around 900 and 1500 ° C., and the sintering times from 1 hour to several hours. It is obvious that soft sintering conditions are sought in order to reduce the tendency of the semiconductor grains to enlarge.

 

   In some cases, electrodes have been formed on the faces of the discs during sintering by applying, on the green disc, paints or pastes to be metallized, containing for example platinum, copper, silver and d other poorly oxidizable metals.



  Table VI which follows summarizes certain operating data and the corresponding results obtained, in terms of properties of the dielectrics. In certain cases, the table indicates the implementation of thermal post-treatments such as cooling in the presence of air after sintering. Such treatment (analogous to annealing) is indicated by the cooling rate in C / h. The second column of the table indicates the number of layers applied by sol-gel on the doped SrTiO3, as well as the type of solution.



   Table VI
EMI7.1


 <tb> Trial <SEP> N " <SEP> Solution <SEP> sol-gel <SEP> = <SEP> <SEP> Sintering <SEP> Properties
 <tb> <SEP> number <SEP> Temp. <SEP> Time <SEP> Post
 <tb> <SEP> from <SEP> layers <SEP> ( <SEP> C) <SEP> (h) <SEP> treatment <SEP> <SEP> E <SEP> Q <SEP>
 <tb> <SEP> H4 (1) <SEP> 1HM, <SEP> SPGI <SEP> 1230 <SEP> 2 <SEP> ¯ <SEP> <SEP> 7760 <SEP> 10.5
 <tb> <SEP> H4 (3) <SEP> 1HM, <SEP> SPGI <SEP> 1263 <SEP> 2 <SEP> - <SEP> <SEP> 5000 <SEP> 14.3
 <tb> <SEP> H4 (4) <SEP> 1HM,

    <SEP> SPGI <SEP> 1248 <SEP> 3 <SEP> 180 <SEP> C / h <SEP> 9000 <SEP> 96.2
 <tb> <SEP> H5 (1) <SEP> SPGI <SEP> 1230 <SEP> 2 <SEP> - <SEP> <SEP> 7000 <SEP> 31
 <tb> <SEP> H5 (2) <SEP> SPGI <SEP> 1248 <SEP> 2 <SEP> - <SEP> <SEP> 7500 <SEP> 9.7
 <tb> <SEP> H5 (3) <SEP> SPGI <SEP> 1263 <SEP> 2 <SEP> - <SEP> <SEP> 5200 <SEP> 17
 <tb> <SEP> H5 (4) <SEP> SPGI <SEP> 1248 <SEP> 3 <SEP> 180 <SEP> C / h <SEP> 6000 <SEP> 99
 <tb>
 The effect which results from the application of an initial layer of aluminosilicate is a reduction in the growth of the grains during sintering, with a somewhat higher combination of the values g and Q. The air cooling increases the factor Q without adversely affecting the permittivity.



     Example 7:
 The materials of Example 6 were brought into play under approximately similar operating conditions. However, before sintering, an additional quantity of liquefiable powdered phase was added to the powder of SrTiO3 reduced and coated by sol-gel, this operation being carried out as follows: the mixture was brought to 1000 C (melting temperature of the glass) in a crucible of alumina a mixture PbO-GeO2 78/22 by weight (called PGO) and, after a waiting period in the molten state (1-4 h), the liquid was poured into molds stainless steel.

  After cooling, the blocks were broken and reduced to powder by grinding according to the technique defined in Example 6 (polyethylene mill, WC / Co balls) to a grain size of about 1-5 m.



   The SrTiO3 powder and a selected proportion of the PGO powder were then dispersed in a 20/80 (V / V) acetone / isopropanol solution containing PVB. From a weight point of view, about 0.5 g of PVB was used per 30 g of solids. The grout was then wrung out and treated and sintered into shapes as in Example 6.



   The results of the measurements carried out on the sintered discs are found in table VII: the titles of the columns of this table refer to the same parameters as those of table VI with, in addition, the quantity by weight of liquefiable phase added (PGO), as well as the nature and the conditions of the air thermal post-treatment: in some cases (C), it is air cooling as already mentioned in Example 6, in other cases (A) , this is annealing at the temperature and for the time indicated.



  Table VII
EMI7.2


 <tb> <SEP> Solution <SEP> Phase <SEP> Sintering <SEP> Properties
 <tb> <SEP> Trial <SEP> sol-gel <SEP> liquefiable <SEP> Temp. <SEP> Time <SEP> Post-processing
 <tb> <SEP> N <SEP> refr. <SEP> (C) <SEP> or <SEP> annealed <SEP> (A) <SEP> s <SEP> Q
 <tb> <SEP> 1 <SEP> layer <SEP>% <SEP> in <SEP> weight <SEP> ("C) <SEP> (h) <SEP> ("C / h) <SEP>
 <tb> H1 (1) <SEP> 1HM <SEP> PGO <SEP> 4% <SEP> 1305 <SEP> 1 <SEP> - <SEP> 1,000,000 <SEP> 0.1
 <tb> H1 (13) <SEP> lHM <SEP> PGO <SEP> 4% <SEP> 1305 <SEP> 1 <SEP> A (1050;

   <SEP> 3) <SEP> 14000 <SEP> 62.2
 <tb> H1 (2) <SEP> îHM <SEP> PGO <SEP> 4% <SEP> 1170 <SEP> 2
 <tb> <SEP> 1270 <SEP> 2 <SEP> - <SEP> 3400 <SEP> 16.2
 <tb> Ho (3) <SEP> lHM <SEP> PGO <SEP> 4% <SEP> 1263 <SEP> 2 <SEP> A (1100;

   <SEP> 1) <SEP> 32000 <SEP> 5.3
 <tb> H3 (1) <SEP> 1HM <SEP> PGO <SEP> 5% <SEP> 1230 <SEP> 2 <SEP> ¯ <SEP> <SEP> 5100 <SEP> 18.3
 <tb> H3 (2) <SEP> 1HM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> 2 <SEP> - <SEP> <SEP> 10800 <SEP> 1,2
 <tb> H3 (3) <SEP> 1HM <SEP> PGO <SEP> 5% <SEP> 1263 <SEP> 2 <SEP> 3700 <SEP> 10.8
 <tb> H3 (4) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1263 <SEP> 2 <SEP> A (1100;

   <SEP> 1) <SEP> 9600 <SEP> 13
 <tb> H3 (5) <SEP> lHM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> 3 <SEP> C (180 C / h) <SEP> 11 <SEP> 600 <SEP> 100
 <tb> H3 (6) <SEP> 1HM <SEP> PGO <SEP> 5% <SEP> 1305 <SEP> 1
 <tb> <SEP> no <SEP> from <SEP> H2 <SEP> 5400 <SEP> 11
 <tb> H3 (7) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> | <SEP> 3 <SEP> C (180 C / h) <SEP> 10650 <SEP> 116
 <tb>
Example 8:
 This example is intended to illustrate the application, after sintering, of a diffusion treatment of a vitreous composition in the molten state inside the ceramic. The doped reduced strontium titanate is the same as in Examples 6 and 7, as well as the operating conditions, until the sintering step of the pressed discs.



   About 0.1 g of a paste prepared with one of the compositions mentioned above at the end of the descriptive part of this application was then applied to the sintered discs. Such a paste was prepared by simply mixing about 3 g of the pulverized solid with a small amount of a solution containing 20 g of PVB in 120 g of n-butanol (taken around 0.1 g of this solution). The discs thus coated were dried and heated for a determined time at the temperature chosen for diffusion. Table VIIIA which follows brings together a first group of parameters and results.

 

  In this case, the grains of doped semiconductor SrTiO3 were prepared by calcining (1100 C, 2 h) a mixture of 1000 g of SrCO3, 541 g of TiO2 and 12.69 g of Nb2Os and reducing it for 4 hours to 1480 C in an H2 / N2 mixture as described above. After spraying as described above, an aluminosilicate layer was applied using the HM9SIA solution (see example 6) and, after the usual treatments, the powder was mixed with 5% (by weight) of PGO (see Example 7); then the sintering was carried out as indicated in Table VIIIA. Finally, the diffusion treatment was applied as indicated in the same table, which also brings together the results of the dielectric measurements of the corresponding capacitors. The electrodes were deposited after spraying.



  Table VIIIA
EMI8.1


 <tb> <SEP> Trial <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100 <SEP> kHz
 <tb> <SEP> N <SEP> Temp. <SEP> Time <SEP> Compo- <SEP> Temp. <SEP> Time <SEP> Gain <SEP> in <SEP> # <SEP> <SEP> Q <SEP> p <SEP> (# <SEP> cm)
 <tb> <SEP> (C) <SEP> (h) <SEP> sition <SEP> (C) <SEP> (h) <SEP> weight <SEP> (%) <SEP> DC
 <tb> H3 (8) <SEP> 1245 <SEP> 3 <SEP> gl <SEP> 940 <SEP> 3 <SEP> 9.4 <SEP> 4000 <SEP> 184 <SEP> 20 <SEP> x <SEP> 109
 <tb> H3 (9) <SEP> 1245 <SEP> 3 <SEP> g2 <SEP> 940 <SEP> 3 <SEP> 11.6 <SEP> 2700 <SEP> 78
 <tb> H3 (11) <SEP> 1206 <SEP> 5 <SEP> gl <SEP> 920 <SEP> 3 <SEP> 3.5 <SEP> 6450 <SEP> 94 <SEP> 21x109
 <tb> H3 (12) <SEP> 1180 <SEP> 6 <SEP> <RTI

    ID = 8.8> gl <SEP> 920 <SEP> 3 <SEP> 1.07 <SEP> 5100 <SEP> 96 <SEP> 94x <SEP> 108
 <tb> H3 (13) <SEP> 1180 <SEP> 6 <SEP> gl <SEP> 920 <SEP> 3 <SEP> 0.78 <SEP> 6000 <SEP> 100 <SEP> 14x <SEP> 109 <SEP>
 <tb>
 In another group of tests, two successive layers of alumina silicate were applied by means of the above-mentioned sol-gel treatment (HM solution), but without additional addition of liquefiable powder phase. In addition, the sintering was carried out without hydrogen, that is to say under pure nitrogen. The operating parameters, the diffusion conditions and the results are recorded in Table VIIIB below:
Table VIIIB
EMI8.2


 <tb> <SEP> Trial <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100kHz <SEP>
 <tb> <SEP> N " <SEP> Temp. <SEP> Time <SEP> Compo- <SEP> Temp.

   <SEP> Time <SEP> Gain <SEP> in <SEP> # <SEP> Q <SEP> P (#. Cm)
 <tb> <SEP> (C) <SEP> (h) <SEP> sition <SEP> (C) <SEP> (h) <SEP> weight <SEP> (%) <SEP> DC
 <tb> H34 (1) <SEP> 1300 <SEP> 3 <SEP> gl <SEP> 930 <SEP> 3 <SEP> 2.36 <SEP> 16800 <SEP> 25.3 <SEP> 27x <SEP> 105
 <tb> H34 (2) <SEP> 1300 <SEP> 3 <SEP> g2 <SEP> 930 <SEP> 3 <SEP> 3.2 <SEP> 14200 <SEP> 27 <SEP> 17 <SEP> x <SEP> 106
 <tb> H34 (3) <SEP> 1284 <SEP> 4 <SEP> gl <SEP> 910 <SEP> 3 <SEP> 3.3 <SEP> 21600 <SEP> 59 <SEP> 83 <SEP> x
 <tb> H34 (4) <SEP> 1284 <SEP> 4 <SEP> g2 <SEP> 910 <SEP> 3 <SEP> 5.3 <SEP> 5650 <SEP> 133 <SEP> 11x109
 <tb> H34 (5) <SEP> 1284 <SEP> 4 <SEP> f <SEP> 940 <SEP> 3 <SEP> 19.2 <SEP> 3450 <SEP> 31 <SEP> 36 <SEP> x <SEP> 107 <SEP>

   
 <tb> H34 (6) <SEP> 1284 <SEP> 2 <SEP> g2 <SEP> 930 <SEP> 3 <SEP> 3.8 <SEP> 11200 <SEP> 30 <SEP> 2x108
 <tb> H34 (7) <SEP> 1284 <SEP> 2 <SEP> gl <SEP> 930 <SEP> 3 <SEP> 3.5 <SEP> 11100 <SEP> 25 <SEP> 25 <SEP> x <SEP> 105
 <tb> H34 (8) <SEP> 1256 <SEP> 4 <SEP> f <SEP> 1200 <SEP> 2 <SEP> 7.7 <SEP> 2800 <SEP> 16 <SEP> 28 <SEP> x <SEP> 108
 <tb> H34 (9) <SEP> 1256 <SEP> 4 <SEP> e <SEP> 1200 <SEP> 2 <SEP> - <SEP> 3000 <SEP> 13 <SEP> 29 <SEP> x <SEP> 108
 <tb>
 A third group of tests similar to the previous ones was carried out, the samples having received a single layer of aluminosilicate sol-gel by means of the HM solution, but no additional liquefiable phase. The discs were sintered under pure nitrogen.



  The results are shown in the table below:
Table VIIIC
EMI8.3


 <tb> <SEP> Trial <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100 <SEP> lcHz <SEP>
 <tb> <SEP> N <SEP> Temp. <SEP> Time <SEP> Compo- <SEP> Temp. <SEP> Time <SEP> Gain <SEP> in <SEP> P <SEP> (# .cm)
 <tb> <SEP> (C) <SEP> (h) <SEP> sition <SEP> (C) <SEP> (h) <SEP> weight <SEP> (%) <SEP> # <SEP> <SEP> Q <SEP> DC
 <tb> H33 (1) <SEP> 1362 <SEP> 2 <SEP> f <SEP> 1100 <SEP> 1.5 <SEP> 17.4 <SEP> 17700 <SEP> 22.3 <SEP> 125x108 <SEP>
 <tb> H33 (8) <SEP> 1300 <SEP> 3 <SEP> gl <SEP> 930 <SEP> 3 <SEP> 1.47 <SEP> 16300 <SEP> 42 <SEP> 12 <SEP> x <SEP> 106
 <tb> H33 (9) <SEP> 1300 <SEP> 3 <SEP> g2 <SEP>

   930 <SEP> 3 <SEP> 1.44 <SEP> 13000 <SEP> 47 <SEP> 93 <SEP> x <SEP> 106
 <tb> H33 (10) <SEP> 1284 <SEP> 4 <SEP> f <SEP> 940 <SEP> 3 <SEP> 21.6 <SEP> 2400 <SEP> 118 <SEP> 164x108 <SEP>
 <tb> H33 (11) <SEP> 1284 <SEP> 4 <SEP> gl <SEP> 910 <SEP> 3 <SEP> 9.4 <SEP> 7700 <SEP> 115 <SEP> 1010 <SEP>
 <tb> H33 (12) <SEP> 1284 <SEP> 4 <SEP> g2 <SEP> 910 <SEP> 3.4 <SEP> 10.9 <SEP> 5800 <SEP> 152 <SEP> 124x <SEP> 108
 <tb>


    

Claims (26)

 CLAIMS  1. Sintered ceramics with grains separated by an insulating barrier layer, usable for the manufacture of multilayer capacitors comprising a granular structure of doped semiconductor SrTiO2 separated by an interstitial insulating phase of metallic compounds, characterized by the fact that the doping metal is chosen among Nb, W and La, that the grains of the granular structure have an average conductivity between 0.1 and 5 Q ncm, an average dimension of 0.5 to 5 llm and that the thickness of the intergranular phase is 100 nm, its permittivity being from 1000 to 100,000 and its Q factor from there 100.  2. Method for obtaining high permittivity ceramics according to claim 1 suitable for the manufacture of high capacity multilayer capacitors, according to which a metal doped SrTiO3 powder is sintered and densified in the presence of a liquefiable sintering agent hot, this process comprising the following steps: :  a) grains of semiconducting powder SrTiO3 doped with tungsten, niobium or lanthanum are mixed with at least one metallic compound capable of functioning as a sintering agent and, thereby, of constituting said insulating barrier layer;  b) the mixture in question is molded under pressure in the form of green articles suitable for the subsequent production of capacitors; ;  c) these green articles are cooked at 1000-13000 C to densify them by sintering, this process being characterized by the fact that said mixture is made (a) by soaking said semiconductor SrTiO3 powder in an organic gelable solution of metal alkoxides, the solution is gelled, so that the grains are charged with a layer of gelled alkoxides, the excess solution is wrung out, the wet powder is dried and the dried powder is heated in order to convert the coating material of said sintering agent.  3. Method according to claim 2, characterized in that said sintering agent is chosen from lead germanate, lead borobismuthate and alumina silicate.  4. Method according to claim 2, characterized in that said solution used in step (a) is chosen from solutions containing lead hexanoate and a germanium alkoxide, boron and bismuth alkoxides, and silicon and aluminum alkoxides.  5. Method according to claim 2, characterized in that said drying of the powder coated in step (a) is carried out by heating in air from 1 to 5 hours at 80-100 "C.  6. Method according to claim 2, characterized in that the dried powder is heated in step (a) 1 to 5 hours at 500-600 "C.  7. Method according to claim 4, characterized in that step (a) is repeated one or more times before undertaking step (b), the solution used during the repetition being identical or different from the previous solution.  8. Method according to claim 2, characterized in that the gain in weight of the semiconductor SrTiO3 powder consecutive to the implementation of step (a) is approximately 0.1 to 1%.  9. Method according to claim 2, characterized in that the average size of the semiconductor doped SrTiO3 grains is largely within the range of 0.5 to 5 lam.  10. Method according to claim 2, characterized in that the average thickness of the intergranular insulating barrier layer of the ceramic resulting from the implementation of step (c) is largely within the range of 0.8 at 10 nm.  11. Method according to claim 2, characterized in that in step (a) is added to the SrTiO3 powder coated with an insulating material by the sol-gel technique a quantity of powderable sintering agent .  12. Method according to claim 11, characterized in that this sintering aid agent is a composition comprising one or more of the following metal oxide compounds: PbO, Gel2, Bizou, H3BO3, B2O3, P2O5 and SrTiO3.  13. Method according to claim 12, characterized in that the said composition is one of the following, by weight: PbO 78% GeO2 28%; PbO 45% - B203 5% - Bi2O3 50%; PbO 25% B202 20% - Bizou 20% - GeO2 5% - BaO 10% - P2O5 20%.  14. The method of claim 2, characterized in that said additional amount consists of 5% by weight of SrTiO3.  15. The method of claim 2, characterized in that the sintered ceramic resulting from step (c) is subjected to a diffusion treatment with a vitrifiable composition, the material thus diffused helping to strengthen or complete the layer- separation barrier between the semiconductor SrTiO3 grains.  16. Method according to claim 15, characterized in that the vitrifiable composition is diffused in the ceramic in the state of liquid or vapor.  17. Method according to claim 15, characterized in that the material to be diffused consists of one or more of the following compounds: PbO, PbF2, Bd203, B203, CuO, GeO2, SrTiO3, BaO.  18. The method of claim 15, characterized in that one adds to the composition to diffuse a metal in divided form, this metal providing, during diffusion, a conductive coating on the outer face of the ceramic so as to achieve a capacitor electrode.  19. The method of claim 18, characterized in that the metal is in the form of a metallization paste and that it is chosen from Cu, Ag, Pt and Au.  20. The method of claim 16 wherein the diffusion is carried out in the liquid state by means of a molten composition, characterized in that the composition is applied to the surface of the ceramic in the form of a grout. comprising said powder composition mixed with an organic binder, then calcining this ceramic in order to decompose the binder and finally baking it to melt the composition and cause it to diffuse in the body of the ceramic.  21. The method of claim 20, characterized in that said slurry still contains a metallic paste, the subsequent firing causing the deposition of this metal in the form of electrically conductive films on the surface of the ceramic body, this film constituting an electrode.  22. The method of claim 16, wherein the diffusion is carried out in the gaseous state, characterized in that the composition to be diffused is melted in a crucible of platinum or alumina and that the object of sintered ceramic in the mouth of this crucible above the level of the molten liquid, so that the vapors from it penetrate into the ceramic.  23. A process for the preparation of doped strontium titanate semiconductor as a starting product in the process according to claim 2, characterized by the fact that mixing and heating either of powdered undoped SrTiO3 with 0.5-1 mol% Nib203, that is TiO2, an equivalent of SrCO3 and 0.5-1 mole% of Nb2Os, and that the mixture is then reduced to 1300-1500 C in the presence of hydrogen and an inert gas or in the presence of CO and mixed CO2.  24. Process for the preparation of semiconductor doped strontium titanate as starting material for the process according to claim 2, characterized in that the powder is quenched. SrTiO3 undoped in an organic gelable solution of niobium alkoxide, which the powder is dried after the gel is deposited there, which is heated and converted into semiconductor SrTiO3 by reduction to 1300-1500 C in the presence of hydrogen or a mixture of CO, CO2 and a non-reactive gas.    25. A method of preparing doped strontium semiconductor titanate as a starting material for the method according to claim 2, characterized in that quenching powdered titanium dioxide in a gelable solution of niobium alkoxide, which the powder containing the gel which has deposited there is dried, mixed with an equivalent of SrCO2 and reduced to 1300-15000 C under an atmosphere of hydrogen and a non-reactive gas or in the presence of a mixture of CO and CO2.  26. A method for preparing doped strontium semiconductor titanate as a starting product of the method according to claim 2, characterized in that it causes the gelation in microparticles of an organic solution of niobium and titanium alkoxides by controlled addition of ammonia, these microparticles are collected, dried and mixed with powdered SrCO3 and the pyrolysis and reduction of the mixture are carried out at 1300 1500 C under an atmosphere of hydrogen and an inert gas or in the presence of a mixture of CO and CO2.