CH669951A5 - Producing semiconductor strontium titanate particles - Google Patents

Abstract

Sr, Ti and dopant metal sources are mixed, where at least one of corresp. organic alkoxides are involved in the mixt. The alkoxide is transformed into gel using the sol.-gel technique. The mixt. is dried and fired in H2, CO or inert gas atmos. Then, the reduced powders are pulverised into the semiconductor SrTiO3 particles having a desired particle size. The high dielectric ceramics are produced as: (a) the semiconductor SrTiO3 particles are mixed with (Nb)SrTiO3 particles which work to lower sintering temp. and to promote insulating grain boundary formation; (b) the mixt is press-moulded; (c) then the mouldings are fired at 1000-13000 deg.C in N2, H2/N2 or CO/CO2 atmos.

Description

       

  
 



   DESCRIPTION



   The present invention relates to doped semiconductor granular materials used to manufacture ceramics with a high dielectric constant, these ceramics being made up of semiconductor grains or particles and of an insulating intergranular layer on the border of these grains. These ceramic compositions are suitable for producing high-capacity bilayer (BL) and multilayer (ML) capacitors. The structure of multilayer capacitors makes it possible to increase their capacity while reducing their size; they are obtained by stratifying, in alternating order, positive and negative laminar electrodes separated by a layer of dielectric and by connecting by edge the electrodes of each group of polarity so that the effective surface of the capacitor corresponds to the sum of the areas of the electrodes of each group.



   The use of grain ceramics limited by a barrier layer (grain boundary barrier layer ceramics) for the manufacture of miniaturized capacitors with high capacity (capacitor
GBBL) has been known for many years. Thus, already in 1960, document GB-A-849-938 mentioned the densification by sintering at 1450 ° C. under hydrogen of grains of BaTiO2 of high purity so as to provide porous semiconductor BaTiO2 having an average particle size of 1 μm. By oxidation in air at atmospheric pressure between 650 and 1000 ° C., an intergranular insulating barrier layer of oxide with a thickness of approximately 10 nm was incorporated into this material.

  The properties of the capacitors obtained from such dielectrics were as follows: effective permittivity x (or ±) approximately 100,000, tangent of the loss angle 3 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 in admixture with materials capable of providing the barrier layer in the form of a liquefiable sintering phase, such materials comprising for example zirconia, tungstate bismuth, lead germanate and others (see, for example, JP-A-77.10595, 1977).

  When sintering is carried out without said liquefiable 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 cooking under an oxidizing atmosphere. 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 baking it at 1300 C, which causes the diffusion of the molten oxide in the ceramic. We thus obtained materials with permittivities on 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, Nb2 Os 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, B2O3, GeO2, P2O5, Au203, MgO, ZnO, Bizou 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 Bi2O2. As optimal electrical properties, permittivity (s) and Q factor values of about 100,000 and 200, respectively, were obtained using SiO2 and Al203 in a weight ratio of about 1.5 to 5.0. The average grain size was in the range of 60 to 120 µm and the resistivity of the interstitial boundary layer in the range of 2 x 109ncm.



   Document JP-A-77-10596 describes the doping of SrTiO2 by mixing with oxides such as Nb2Os, Ta2Os, WO3 or TiO2 and baking at around 1400 C under a reducing atmosphere. An amount of about 1% by weight of doping agent is sufficient to obtain resistivities of less than 10 μl Q cm. This reference also mentions the use of SiO2 as a sintering additive, this material favoring the magnification of semiconductor grains during 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 SrTiO3 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 doped semiconductor powder, for example niobium, used as internal granular phase in the present process can be obtained by doping SrTiO3 with a doping metal compound, for example by mixing SrTiO3 and Nb2Os and baking the mixture under a reducing atmosphere, for example at 1450 1500 C in the presence of an H2 / Ar or N2 / N2 mixture containing (V / V) 1-10% of H2. Alternatively, a ground mixture of TiO2, Nb2Os and SrCO2 can also be subjected to such cooking. After doping, the material is ground to the chosen particle size (corresponding to grains with a size of 0.5 to 5 μm) by the usual means, for example in a ball mill in the presence of ceramic or hard metal balls. .

  Other doping metals can be introduced similarly, for example tungsten and lanthanum.



   Such a technique is generally suitable for obtaining semiconductor grains of a certain size (in particular around 10 μm or more) usable in the case of the manufacture of BL capacitors. However, it is less suitable for the smaller grains (for example less than 10 μm or less) that are used for the ML capacitors. Indeed, the effective permittivity (or effective dielectric constant) of barrier layer ceramics comprising insulating layers at the periphery of the semiconductor grains depends on the grain size and the thickness of the intergranular insulating layer. The relationship providing the capacitance of a capacitor according to the properties of the ceramic and its dimensions is as follows (see for example Ultrahigh 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 constant intrinsic dielectric 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 nm (0.1 to 1 μm).

 

   However, as already explained above, the dielectrics whose grains exceed about 5-10 μm are not suitable for ML capacitors whose thickness of the layers is 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 Internal Boundary Layer Ca pacitors 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 mm (0.1 pm). We thus obtained effective permittivities in the 103 range with relatively few dielectric losses (tg 6 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 tg 8 represents the dissipation factor, 6 being the difference (in angular units) between 90 and the voltage-intensity phase shift angle 9 of the alternating current applied to the capacitor. Ideally, if the capacitor shows no loss, 9 would be 3: / 2 and 6 would be zero. Q is the inverse of tg 6; Q = l / tg 6.



   However, it was desirable to make further improvements to the results of the known technique and to provide doping techniques leading to grains of reduced size, homogeneously doped, not requiring intensive grinding, but providing ceramics of high permittivity. without conceding too many losses with regard to thermal stability and the Q factor. The process of the present invention, as defined in claim 1, constitutes a step forward in the desired direction. The definition of the Sol-Gel technique is given below in this description.



   Consequently, according to an advantageous technique for preparing SrTiO3 according to the invention, the SrTiO3 powder is impregnated or mixed with an organic solution (of Sol-Gel type) of an alkoxide of a doping metal under the protection of a inert gas (for example, pure nitrogen or argon). 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, water and a gelling catalyst can be incorporated into the solution, for example small amounts of HCl or other mineral acid; bases such as amines or NH40H 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, it is possible, for example, to heat from 400 to 1000 ° 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.

  Thus, the undoped SrTiO3 powder can be impregnated with a solution of niobium alkoxide in an organic solvent, then it is wrung, dried at 90 "C and heated in air until 600 C at the speed of a few C / min (4 C / min are suitable) Then a cooking-reduction operation is carried out by heating for example 2 hours at 1450-1500 "C in a reducing atmosphere, for example a mixture of 1 at 10% (V / V) H2-Ar, H2-N2 or CO
CO2. To then obtain a doped powder of desired particle size, the reduced material is ground in a ball mill to the desired grain size, for example from 0.5 to 5 μm.



   According to another embodiment, the above Sol-Gel treatment is applied to TiO2 and this, after doping with niobium and possible reduction, is mixed with strontium carbonate, the resulting mixture then being reduced to 1300 -1500 C under conditions similar to those indicated above, then ground to the desired particle size.



   According to another embodiment, it is possible to gel Ti and Nb alkoxides in solution by adding ammonia, the gel is separated and dried to reduce it to a fine powder of TiO 2 doped with niobium; this powder is then mixed with
SrCO3, and the mixture reduced as above in an atmosphere containing hydrogen or carbon monoxide.



   According to a variant of the above embodiment, implying moderate conditions of heat treatment and, consequently, a lesser tendency to the enlargement of the grains and to their agglomeration during said treatment, a gelable solution of titanium alkoxides is atomized. and niobium (by means of a spray gun) and the particles resulting from this atomization are sprayed into an aqueous ammonia solution with rapid stirring (1- 10% NH40H in water), which causes gelation instant of these solid particle droplets.

  These are then separated by filtration or centrifugation, air dried, gradually raising the temperature to about 400 C (1- 5 "C / min) and the dried particles are mixed with the appropriate amount of strontium. (in the form of SrCO), then the whole is slowly heated again in air to 1000 C., but without exceeding this temperature. This thermal pretreatment with air has the effect of significantly reducing the temperatures required for final reduction.



  In fact, this reduction can then be carried out at a temperature which does not substantially exceed 1000 ° C. in a reducing atmosphere, that is to say a mixture of hydrogen or carbon monoxide and an inert gas such as nitrogen. , carbon dioxide, argon, helium or any rare gas, with a reducing gas / carrier gas ratio of about 1/99 to 10/90. The effect produced by this succession of heat treatments at temperatures lower than those of the preceding embodiments consists in a marked reduction in the magnification of the grains of the powder and in the tendency of the latter to agglomerate during the treatment and, therefore, at much milder grinding conditions to reach the final particle size.

  Thus, the grinding times decrease from approximately 10-20 h to approximately 1-2 h or less and particles of approximately 1 μm or less are obtained, this average dimension being much more homogeneous; by this we mean that the relative proportion of particles above or below the mean value is much lower.



  Under such conditions, ceramics with high dielectric power are obtained from these particles in a more homogeneous and reproducible manner. The doped SrTiO powder can also be prepared as above, but using a solution of Ti, Ni and Sr alkoxides in pre-established molar proportions.



   The strontium titanate doped with Sol-Gel according to the present invention can be advantageously used for the preparation, by sintering, of high permittivity ceramics. Although, to do this, the powder can be molded into a suitable form, under pressure without additives, it is preferable to work in the presence of a sintering assistance phase, which contributes to reducing the magnification of the grains during this sintering and to providing finally (in whole or in part) the required boundary layer of intergranular material. These conditions are particularly desirable in the case of multilayer capacitors in which the grains must remain small (below 5-10 μm), preferably in the micron range.



   The agents constituting the liquefiable phase of sintering aid may be chosen from the metal oxides generally used for the manufacture of capacitors with an intergranular barrier layer. Such materials can be used in the form of pulverulent compositions of metal oxides in the solid state which are mixed mechanically. They can also be added by the Sol-Gel technique, this asepct constituting a new factor which completes the present invention. 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 an insulating barrier layer is formed. In other words, the amount 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.

  So,
The effective thickness of the interstitial layer coating the grains can be as small 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 which is obtained as described above 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 (ie 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 in a similar manner to that used previously for its doping, that is to say by mixing the latter (the grains of which have a dimension of approximately 0.8 to 5 μm) under an inert atmosphere (for example of 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 NH40H 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, it is possible, for example, to 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.



   The metal compounds which can be used to form the solutions of alkoxides are numerous and, among these, 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. For this purpose, 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 which can be decomposed subsequently. (as a binder, a volatile or easily combustible polymer such as polyvinyl alcohol or polyvinyl butyral can be used).

  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, Bi2O ,, H3BO ,, BaCO ,, P2O5 and SrTiO3. The use of SrTiO3 as a component of the liquid sintering phase during sintering of the semiconductor doped SrTiO grains is new and provides notable advantages with regard to the stabilization of the insulating intergranular phase, the permittivity and the 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% - Bi2O3 50%; PbO 25% - B2O3 20% - Bi2O, 20% - GeO2 5% - BaO 10% - P2O5 20%.



   When SrTiO 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 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 high interstitial layer of high dielectric constant, first of all comprises 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 13000 C under inert gas such as argon or nitrogen or in a reducing atmosphere so as not to risk losing the semiconductor properties of doped SrTiO.

  As the reducing atmosphere, dilutions of hydrogen or carbon monoxide can be used in an inert gas such as nitrogen or a noble gas. Permittivities of the order of 106 were thus obtained accompanied by low Q factors. By a suitable 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 using a piston press (2-5 tonnes / cm2), the pressed discs having about 10-20 mm in diameter and 1-3 mm in thickness, by carrying out the sintering 1-6 h at a given temperature, by fixing electrodes on both sides 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-I2 F):
 C = 0.0885 g 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 vacuum spraying by known techniques or by screen printing using a paint or 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 laying electrodes; thus, the powdery vitrifiable oxides were kneaded with a paste or 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 into the ceramic while the metal is deposited on the surface thereof; thus providing the desired electrodes.



   Some of the preferred compositions for carrying out the aforementioned 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% - Bi2O3 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% - Bi2O3 20% - B203 40% - CuO 20%
 (i) PbO 66% - GeO2 12% - Bi2O3 20% - B203 2%
 Alternatively, the vapors of said molten oxides were used for diffusion.

  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 approximately 1-3 cm 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 B2O3 / PbO (temperature of the molten liquid 900 C).



   The following examples illustrate the invention in detail.



  Example I
 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 '' 0.1N ethanolic solution).



   About 15 g of very pure strontium titanate powder (Kyorix 1150 / A) was impregnated with this solution at room temperature, then, after gelation, the product was drained and heated for 2 h at 600 C (at the 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% mixture. (V / V) of hydrogen and argon. A semi-conductive powder (STK-III) was obtained which was ground as in
 previous examples; its resistivity, measured by the means
 tuels, was 1.43 ohm cm.



   The coating of the grains of this powder was carried out by soaking in a solution (PG2) containing 12 g of acetylacetone, 1.06 g
 of Ge (OEt) 4, 2.74 g of ethyl-2-hexanoate of Pb, 23.32 g of n-butanol
 and 268.5 ml of 0.1N ethanolic Hui solution (0.98 g HUI).



  After impregnation and gelation, the powder was drained, dried and then heated in air for 2 hours 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 I below, Ci also indicating the dielectric properties of sintered discs.



  Example the
 A semiconductor powder of SrTiO, (STK-I) was prepared as in Example I but using, for doping, a solution (SI) containing 5 g of acetylacetone, 2.39 g of ethoxide niobium, 12.1 g of ethanol and 0.6 g of HCl (0.1N ethanolic solution). After the impregnation carried out as in Example 1, 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 SrTiO3 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 GoO2. 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 in air, after which the block was reduced to powder and crushed, as explained before, in a plastic ball mill in the presence of WC balls and a mixture 3: 1 (V / V) petroleum ether and tbutanol, to a particle size of about 0.5-5 µm. The sintering in the form of test discs was carried out as summarized in Table I in which the electrical properties are recorded for comparison.

  The term flash used in Table I indicates that the sample was quickly heated to operating temperature.



  Table I
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    <SEP> (C / min) <SEP> (C) <SEP> (min) <SEP> # <SEP> Q
 <tb> <SEP> C4-L <SEP> N2 <SEP> 7 <SEP> 1100 <SEP> 60 <SEP> 3522 <SEP> <SEP> - <SEP>
 <tb> <SEP> C4-M <SEP> M <SEP> <SEP> N2 <SEP> flash <SEP> 1114 <SEP> 33 <SEP> 1788 <SEP> <SEP> - <SEP>
 <tb> Test <SEP> C4T <SEP> N2 <SEP> 7 <SEP> 1250 <SEP> 120 <SEP> 10300 <SEP>
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   <SEP> 3 <SEP> C4-U <SEP> N2 <SEP> 7 <SEP> 1000 <SEP> 270
 <tb> <SEP> 1060 <SEP> 120 <SEP> 3374 <SEP> 14.3
 <tb> <SEP> V <SEP> N2 <SEP> 6 <SEP> 1250 <SEP> 120 <SEP> 5762 <SEP> 3.9
 <tb> <SEP> 04-W <SEP> N2 <SEP> 6 <SEP> 1250 <SEP> 120 <SEP> 7155 <SEP> 4.8
 <tb> <SEP> C3-K <SEP> N2 <SEP> 7 <SEP> 1100 <SEP> 60 <SEP> 2839 <SEP> <SEP> - <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 dielectric properties of the samples for which the intergranular insulating phase was obtained by Sol
Gel are superior to those of the samples where this insulating liquefiable phase was obtained by mechanical mixing of powders and that, moreover, the application of three layers of material intended to provide the intergranular barrier gives superior results to the application of 5 layers.

 

  Example 2
 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 0.5N aqueous NH40H solution and alcohol to make 626 ml.



   The mixture resulting from the addition of B to A had the following titers: 0.1M titanium and 0.59M water; it also contained 0.054 mole of ammonia. The mixture provided a fine precipitate and, after 1 hr stirring, 830 ml of water was added and stirring was continued 1/4 hr. Then, after having left to stand for 16 h, the clear liquid was siphoned off and replaced with 1 liter of alcohol. The precipitate was separated by filtration and redispersed for 1 hour in 1 liter of boiling alcohol at reflux. The precipitate I 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. It was then reduced to obtain a semiconductor product; the operation was carried out as already described by 4 hours of heating at 1480 ° C. under a 10% hydrogen / nitrogen mixture (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 drying in air for 4 hours at 90 "C, the powder was finally heated for 1 hour at 550" 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 by means of the solution defined as PGI (see example 3). We applied 4 successive layers with, between the layers, a drying operation of 4h at 90 "C; then we gradually heated (4 C / min) to 550" C, temperature maintained 1 h 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 set out in Table II; 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 II
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 <tb> <SEP> Sintering <SEP> Properties <SEP> dielectrics
 <tb> Samples
 <tb> atm. <SEP> heating <SEP> temp. <SEP> time <SEP> ± <SEP> Q <SEP>
 <tb> <SEP> ( <SEP> C / min) <SEP> (O <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> 9940 <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 in the first period to 1200, then, in the second period, lh to 13500C.



   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 soaking it in a niobium hydroxide solution, this operation being followed by the same treatment, described above, used to boost SrTiO3 by Sol-Gel (see example 3). Then mix the niobium-treated TiO3 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 3
 923.78 g of strontium carbonate (6.26 moles) and 500 g of TiO2 (6.26 moles) were dispersed under nitrogen in a solution containing 14 g of Nb (EtO) 5 (0.044 moles), 40 g of acetylacetone, 100 g of n-butanol and 4 mg of HCl (added as 1N ethanolic solution). After standing for 3 hours at room temperature, the liquid was separated from the dispersion by wringing and sprayed in a zirconia mill with water as the carrier phase and 1 kg of ZrO2 beads. As the carrier phase, water was used. The powder cake was dried in air and then heated for 2 hours at 1100 ° C. in air in an alumina crucible (rate of temperature setting, 400 ° C./h).



  A reduction was then carried out by heating 2-4 h at 1450 1480 C under a 10/90 (V / V) mixture of hydrogen / nitrogen. The heating rate was about 300-420 "C / h. Then the reduced product was ground in a polyethylene mill in the presence of beads
WC / Co up to a particle size of 0.5 to 5 * um. 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 1/2 h at 70 "C 41.6 g of Si (EtO) 4.30 g of n-butanol, 7.9 g of di-sec-butoxydeacetylacetate chelate aluminum and 1.84 g of HCl (in the form of a 0.1N solution in ethanol). After cooling, 3.3 g of H2O and 182 g of EtOH were added. V2 was again heated h 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 is heated to reflux 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 from 60 to 240 C / h depending on the case. During this treatment, all organic residues are destroyed and eliminated; if necessary, this treatment can be carried out in oxygen to accelerate the combustion processes. The powder thus densified 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 / nbutanol.



   After having stirred for 30 min, it is evaporated, the powder coated with PVB is passed through a 300 μm mesh screen and it is pressed in the form of discs in a stainless steel mold at 25 tonnes / 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 Au203 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 takes place, 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 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, to the green disc, paints or pastes to be metallized, for example containing platinum, copper, silver and other metals. not very oxidizable. Table III 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 SrTiO2 as well as the type of solution.



  Table III
EMI7.1


 <tb> <SEP> Sintering <SEP> Properties
 <tb> Trial <SEP> N0 <SEP> Solution <SEP> Sol-Gel;
 <tb> <SEP> number <SEP> from <SEP> oouches <SEP> temp. <SEP> time <SEP> <SEP> post-processing <SEP> o <SEP> Q <SEP>
 <tb> <SEP> ( <SEP> C) <SEP> (h)
 <tb> <SEP> H4 (1) <SEP> lHM, <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> lHM,

    <SEP> 5PGI <SEP> 1248 <SEP> 3 <SEP> 180 / h <SEP> 9000 <SEP> 96.2
 <tb> <SEP> H5 (1) <SEP> 5PGI <SEP> 1230 <SEP> 2 <SEP> 7000 <SEP> 31
 <tb> <SEP> H5 (2) <SEP> 5PGI <SEP> 1248 <SEP> 2 <SEP> - <SEP> <SEP> 7500 <SEP> 9.7
 <tb> <SEP> H5 (3) <SEP> 5PGI <SEP> 1263 <SEP> 2 <SEP> - <SEP> <SEP> 5200 <SEP> 17.0
 <tb> <SEP> H5 (4) <SEP> 5PGI <SEP> 1248 <SEP> 3 <SEP> 1800 / h <SEP> 6000 <SEP> 99
 <tb>
 The effect which results from the application of an initial layer of alumi silicate is a reduction in the growth of the grains during sintering with the consequence of a somewhat greater combination of the values ± and Q. Air cooling increases the Q factor without adversely affecting the permittivity.



  Example 4
 The materials of Example 3 were brought into play under approximately similar operating conditions. However, before sintering, reduced and soil-coated SrTiO3 powder was added
Gel an additional quantity of liquefiable phase in powder, this operation being carried out as follows: a PbO-GeO2 mixture 78/22 by weight (referred to as PGO) and, after a waiting period in the molten state (1-4 h), the liquid was poured into stainless steel molds. After cooling, the blocks were broken and reduced to powder by grinding according to the technique defined in Example 3 (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 3.



   The results of the measurements carried out on sintered discs are found in table IV: the titles of the columns of this table refer to the same parameters as those of table III with, in addition, the quantity by weight of liquefiable phase added (PGO) as well as the nature and conditions of the air thermal post-treatment: in some cases (C), this is air cooling, as already mentioned in Example 3; in other cases (A), it is annealing at the temperature and for the time indicated.



  Table IV
EMI7.2


 <tb> <SEP> Sintering <SEP> Properties
 <tb> <SEP>. <SEP> Solution <SEP> Phase <SEP>
 <tb> <SEP> Naked <SEP> Sol-Gel <SEP> liquefiable <SEP> temp <SEP>. <SEP> <SEP> time <SEP> post-processing
 <tb> <SEP> 1 <SEP> layer <SEP>% <SEP> in <SEP> weight <SEP> (C) (H2 (h) <SEP> refr.

   <SEP> (C) <SEP> or <SEP> re- <SEP> # <SEP> <SEP> Q
 <tb> <SEP> cooked <SEP> (A) <SEP> (C, h) <SEP>
 <tb> Hl (l) <SEP> îHM <SEP> PGO <SEP> 4% <SEP> 1305 <SEP> 1 <SEP> - <SEP> 1,000,000 <SEP> 0.1
 <tb> H1 (13) <SEP> îHM <SEP> PGO <SEP> 4% <SEP> 1305 <SEP> 1 <SEP> A (1050; <SEP> 3) <SEP> 14000 <SEP> 62.2
 <tb> H1 (2) <SEP> 1HM <SEP> PGO <SEP> 4% <SEP> 1170 <SEP> 2
 <tb> <SEP> 1270 <SEP> 2 <SEP> - <SEP> 3400 <SEP> 16.2
 <tb> H1 (3) <SEP> îHM <SEP> PGO <SEP> 4% <SEP> 1263 <SEP> 2 <SEP> A (1100;

   <SEP> 1) <SEP> 32000 <SEP> 5.3
 <tb> H3 (1) <SEP> lHM <SEP> PGO <SEP> 5% <SEP> 1230 <SEP> 2 <SEP> - <SEP> <SEP> 5100 <SEP> 18.3
 <tb> H3 (2) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> 2 <SEP> - <SEP> <SEP> 10800 <SEP> 1,2
 <tb> H3 (3) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1263 <SEP> 2 <SEP> - <SEP> <SEP> 3700 <SEP> 10.8
 <tb> H3 (4) <SEP> lHM <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 / h) <SEP> 11600 <SEP> 100
 <tb> H3 (6) <SEP> lHM <SEP> PGO <SEP> 5% <SEP> 1305 <SEP> 1
 <tb> <SEP> no <SEP> from <SEP> H2 <SEP> ¯ <SEP> 5400 <SEP> 11
 <tb> H3 (7) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> <SEP> 3 <SEP> C (1800 / h) <SEP> 10650 <SEP> 116 <SEP>
 <tb>
Example 5
 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 3 and 4 as well as the operating conditions, this up to 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. The following table VA 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 Nb205 and reducing it 4 h 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 3) and, after the usual treatments, the powder was mixed with 5% (by weight) of PGO (see Example 4); then the sintering was carried out as indicated in table VA. 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.



  VA table
EMI8.1


 <tb> <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100 <SEP> kHz
 <tb> <SEP> Trial
 <tb> <SEP> Trial <SEP> temp. <SEP> time <SEP> compo- <SEP> temp. <SEP> time <SEP> gain <SEP> in <SEP> Q <SEP> p (Q <SEP> p (fl <SEP> cm) <SEP>
 <tb> <SEP> (OC) <SEP> (h) <SEP> sition <SEP> ( <SEP> <SEP> C) <SEP> (h) <SEP> weight <SEP> (%) <SEP> ± <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 <SEP> 25 <SEP> x <SEP> 109
 <tb> H3 (11) <SEP> 1206 <SEP> 5 <SEP>

     gl <SEP> 920 <SEP> 3 <SEP> 3.5 <SEP> 6450 <SEP> 94 <SEP> 21 <SEP> x <SEP> 109
 <tb> H3 (12) <SEP> 1180 <SEP> 6 <SEP> gl <SEP> 920 <SEP> 3 <SEP> 1.07 <SEP> 5100 <SEP> 96 <SEP> 94 <SEP> x <SEP> 108
 <tb> H3 (13) <SEP> 1180 <SEP> 6 <SEP> gl <SEP> 920 <SEP> 3 <SEP> 0.78 <SEP> 6000 <SEP> 100 <SEP> 14 <SEP> x <SEP> 109
 <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 VB below:
Table VB
EMI8.2


 <tb> <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100 <SEP> kHz
 <tb> <SEP> Trial
 <tb> <SEP> " <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> n <SEP> x <SEP> loS <SEP>
 <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 <SEP> 10 '
 <tb> H34 (4) <SEP> 1284 <SEP> 4 <SEP> g2 <SEP> 910 <SEP> 3 <SEP> 5.3 <SEP> 5650 <SEP> 133 <SEP> 11 <SEP> x <SEP> 10, <SEP>
 <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> 2 <SEP> x <SEP> 10 '
 <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> 100 <SEP>
 <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> 10 '
 <tb> H34 (9) <SEP> 1256 <SEP> 4

    <SEP> e <SEP> 1200 <SEP> 2 <SEP> - <SEP> 3000 <SEP> 13 <SEP> 29 <SEP> x <SEP> 10 '
 <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 VC
EMI8.3


 <tb> <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100kHz <SEP>
 <tb> <SEP> Trial
 <tb> <SEP> N <SEP> temp. <SEP> time <SEP> compo- <SEP> temp. <SEP> time <SEP> gain <SEP> in <SEP> E <SEP> <SEP> Q <SEP> <SEP> p (Rcm) <SEP>
 <tb> <SEP> ( <SEP> C) <SEP> (h) <SEP> sition <SEP> eC) <SEP> (h) <SEP> weight <SEP> (%) <SEP> ¯ <SEP> <SEP> DC
 <tb> H33 (1) <SEP> 1362 <SEP> 2 <SEP> @ <SEP> <SEP> 1100 <SEP> 1.5 <SEP> 17.4 <SEP> 17700 <SEP> 22.3 <SEP> 125 <SEP> x <SEP> 102 <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> 164 <SEP> x <SEP> 10 '
 <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> 124 <SEP> x <SEP> 10 '
 <tb>


    

Claims (23)

 CLAIMS  1. Process for the preparation of powdered semiconductor SrTiO3 usable in the manufacture of high permittivity dielectrics with grains limited by an intergranular barrier layer suitable for the manufacture of high capacity capacitors, this process comprising the use of powdered SrTiO2 which is doped with metals chosen from Nb, La, W and Co by subjecting mixtures of compounds of Sr, Ti and the doping metal to a heat treatment in a slightly reducing atmosphere to reduce the doping metal compound in its form elementary, characterized in that the said compounds of Sr, Ti and the doping metal are brought into contact with each other, at least one of these compounds being an organic alkoxide, this alkoxide is converted into a gel, this gel being mixed with non-alkoxide compounds,  this mixture is dried and calcined under a reducing atmosphere containing hydrogen or carbon monoxide and a non-reactive gas and, finally, the reduced product is pulverized until a predetermined particle size is reached.  2. Method according to claim 1, characterized in that quenching of powdered SrTiO3 in a gelable solution of niobium alkoxide, the wet powder is separated by wringing, it is dried in air and it is calcined at 1300-1500 C in a container atmosphere H2 or CO.  3. Method according to claim 1, characterized in that the TiO2 powder is soaked in a gelable solution of niobium alkoxide, the wet powder is wrung, it is dried in air, it is mixed with an equivalent of powdered SrCO3 and the whole is calcined at 1300-1500 ° C. in an atmosphere containing H2 or CO.  4. Method according to claim 1, characterized in that a gelable solution of titanium alkoxides and niobium is added to an aqueous ammonia solution, the latter being agitated in such a way that when the solutions meet , that containing the alkoxides disperses into microscopic particles hardening by gelation, these particles are separated and dried, they are mixed with an equivalent of SrCO3 and the whole is calcined in a reducing atmosphere containing hydrogen or carbon monoxide.  5. Method according to claim 4, characterized in that the atomization of the titanium and niobium alkoxides solution in the ammonia solution with stirring, then, after having separated them, the particles are air dried which have formed by heating them slowly to about 400 "C, they are mixed with the required amount of SrCO3, this mixture is slowly heated in air to 1000 "C but without significantly exceeding this temperature, this treatment inducing the formation of grains of mixed oxides of strontium-titaneniobium, these grains having less tendency to grow in heat then the reductive cooking treatment is carried out in a mixture of hydrogen or carbon monoxide and an inert gas such as nitrogen,  carbon dioxide or a rare gas at a temperature practically not exceeding 10000 C, which causes a significant reduction in the growth of the grains with heat and allows the final grinding to be carried out under milder conditions and providing a product of more homogeneous particle size.  6. Method according to claim 1, characterized in that the reducing atmosphere in the final step consists of a mixture of hydrogen and a rare gas or nitrogen, or carbon monoxide and dioxide, and that this step is carried out at temperatures of about 950 to 1310 "C.  7. Use of the doped SrTiO3 obtained by the process according to one of claims 1 to 5 in the manufacture by sintering of high-permittivity dielectrics with grains limited by an intergranular barrier layer suitable for capacitors of high capacity, ca reacted by the fact than  (a) the grains of said doped SrTiO3 (Nb) are mixed with at least one solid mineral compound capable of forming a liquefiable phase promoting sintering by reducing the temperature necessary for this and by providing said internal insulating barrier layer;  (b) this mixture containing SrTiO3 is pressure molded in the form of sinterable ceramic bodies usable as dielectrics of capacitors;  (c) these bodies are sintered at 1000-1300 C under an atmosphere of nitrogen or of H2 / N2 or CO / CO2 mixtures.  8. Use according to claim 7, characterized in that step (a) is carried out by mechanically mixing the SrTiO3 doped with a mineral composition for sintering aid in powder form, this composition containing at least one oxide of the following: Pb, Ge, Bi and Ba.  9. Use according to claim 7, characterized in that step (a) is carried out by the technique according to which the grains of powdered doped SrTiO3 are impregnated with metal alkoxides in solution, these alkoxides are converted into a gel so as to coat these grains with a layer of metalloxane material, the powder thus coated is dried and calcined in air so as to transform the coating into said solid sintering aid phase, which then consists of compounds d 'metal oxides.  10. Use according to claim 9, characterized in that the said alkoxides are chosen from Pb, Ge, Bi and Ba alkoxides, the metal oxide compounds of the phase promoting sintering comprising, respectively, lead germanate and lead and boron bismuthate.  11. Use according to claim 9, characterized in that in step (a) a preliminary coating of alumina silicate is deposited on the doped SrTiO3 grains, this deposit being produced by means of gelable solutions containing alkoxides silicon and aluminum.  12. Use according to claim 8, characterized in that the doped SrTiO3 comprises a prior coating of metal oxide as obtained according to claims 8 to 10.  13. Use according to claim 7, characterized in that the bodies once sintered are subjected to a thermal post-treatment with air.  14. Use according to claim 13, characterized in that said post-treatment consists in air cooling said still hot bodies according to a pre-established cooling plan.  15. Use according to claim 13, characterized in that after cooling under an inert or reducing atmosphere, said sintered bodies are subjected to annealing with air or with oxygen.  16. Use according to claim 14, characterized in that the sintered body is cooled from the sintering temperature to ambient temperature at a speed of 50 to 250 C / h.  17. Use according to claim 14, characterized in that the sintered body is first cooled from the sintering temperature to a pre-established temperature of lower degree and that, thereafter, its controlled air cooling is carried out at a speed of 50 to 2,500C / h.  18. Use according to claim 17, characterized in that said preset temperature is from 800 to 1100 C and that said cooling rate is from 150 to 200 C / h.  19. Use according to claim 15, characterized in that after its initial cooling, the sintered body is heated in air or under oxygen at a temperature of 800 to 12,000 C from 0.5 to Sh.    20. Use according to claim 19, characterized in that before heating the sintered body, a vitrifying composition is applied to the surface of the sintered body, this composition melting at the temperature of the annealing and diffusing into the body of ceramic so as to strengthen the insulating intergranular boundary phase.    21. Use according to claim 20, characterized in that the said vitrifiable composition consists of a grout or paste of powdered mineral matter applied to the surface of the sintered body.  22. Use according to claim 21, characterized in that the said vitrifiable composition comprises at least one of the following compounds: PbO, GeO2, B203, Bu203, PbF2, CuO, SrTiO3.  23. Use according to claim 21, characterized by the  the fact that said paste also contains a metal in the divided state, this metal forming a metallic film on the ceramic surface during diffusion and being subsequently usable as a capacitor electrode.