CH667643A5 - High dielectric internal grain boundary insulating layer ceramic - Google Patents

Abstract

Metal coating is applied to the external circumference of doped titanic acid strontium grain. The grain is then moulded, sintered and heated.

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 even or odd electrodes two by two, alternately, the even (and odd, respectively) electrodes being connected together by the edge so that the total of the areas of the elements of each group constitutes the surface total effective of the capacitor.



   The use of grain ceramics limited by a barrier layer (grain boundary barrier layer ceramics) for the manufacture of high capacity miniaturized capacitors (GBBL capacitors) 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 PaTiO2 grains of high purity so as to provide porous semiconductor PaTiO2 having an average particle size of 1 μm. By oxidation in air at atmospheric pressure between 650 and 10000 C, an intergranular insulating barrier oxide layer 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 e) 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 liquefiable sintering phase, such materials comprising for example zirconia, bismuth tungsten , lead germanate and others [see for example JP-A-77.10595 (1977) 1. When the 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 firing under an oxidizing atmosphere in the body of the ceramic (see BP-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 Bizou 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, 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 six2, P2O3, GeO2, P2Os,
Al203, MgO, ZnO, Bizou and TiO2.



   Document JP-A-83-97820 relates to the preparation of semiconductor ceramics by pressure molding and sintering at 13501450 "C under a 1:99 atmosphere of hydrogen and nitrogen of mixtures of SrTiO3, WO3, Ge2O, Al203. To constitute the insulating barrier layer, a molten mixture of
PbO, B203 and Bi2O2. As optimal electrical properties, permittivity (E) and Q factor values of about 100,000 and 200 were obtained, respectively, using SiO2 and Au203 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 109 µl.



   The 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 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 11 cm. This reference also mentions the use of SiO2 as a sintering additive, this material promoting the enlargement of the semiconductor grains in sintering and obtaining grains of a size greater than 10 μm having the 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 the diffusion of a melt mixture of PbO, B203 and Bi2O3. As typical values of 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 grain size and on the thickness of the intergranular insulating layer. The relationship providing the capacitance of a capacitor as a function of the properties of the ceramic and its grain 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 Interfaces 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 ttm and separation layers of 102 to 103 nm (0.1 to 1 pm).



   However, dielectrics whose grains exceed approximately 510 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 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 was of the order of 100 nm (0.1 μm). We thus obtained effective permittivities in the 103 range with relatively few dielectric losses (tangent 3 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, 6 being the difference (in angular units) between 90 "and the angle e of phase-shift voltage-intensity of the alternating current applied to the capacitor.

  Ideally, if the capacitor does not show any loss, e would be equal to x / 2 and 6 would be equal to zero. Q is the inverse of tangent o;
Q = I / tangent 6.



   It was however desirable to bring further improvements to the results of the known technique and to provide ceramics of even higher 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; this step was taken when the present inventors discovered with surprise that certain treatments at high temperature in the presence of air after sintering provided beneficial effects on the dielectric properties of the capacitors; indeed, depending on the operating conditions used, one could normally expect an alteration of the semiconductor properties of the SrTiO3 grains by overoxidation.



   The conditions under which the treatments according to the invention can be carried out are detailed below: in the case where an annealing is carried out, the sintered object is heated in air in an oven at a predetermined temperature and for a time of variable duration. The temperatures are of the order of 800 to 1100 "C and the annealing times of the order of 0.5 to 5 h.



   When you just cool the sintered object with air, you replace the gas in which the sintering was performed (usually nitrogen and hydrogen) with air. The temperature at which this change takes place may be equal to or lower than the sintering temperature; air is usually introduced around 1100-1200 "C or, preferably, below, that is to say around 1050 or 1000" C, or even lower, while the sintering itself takes place between 1200 and 1400 "C. The cooling rate under these conditions is of the order of about 100 to 200" C / h. In some cases, the annealing treatment has been combined with diffusion post-processing, as will be seen below.



   The doped semiconductor powder, for example niobium, used as an internal granular phase in the present process can be obtained by mixing SrTiO2 and Nb2Os and baking the mixture under a reducing atmosphere, for example at 1450-1500 "C in the presence of an H2 / Ar or H2 / N2 mixture containing (V / V) 1-10% H2. Alternatively, it is also possible to subject to such a cooking a ground mixture of TiO2, Nb2Os and SrCO3. After doping, it is ground the material with 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.

 

   When SrTiO3 doped with the desired particle size is available, the grains are mixed with the materials constituting the liquefiable aid for sintering and densification, these materials also subsequently providing, after cooking, the limit phase insulating intergranular desired. Such materials can be used in the form of pulverulent compositions of metal oxides in the solid state which are mixed mechanically.



   In another way, these materials can be supplied in the form of organometallic compounds, usually alkoxides, which are gelled in contact with SrTiO3, a coating or film of metalloxane structure then covering the grains thereof. This technique constitutes an additional new feature of the invention. Usually this addition is done by impregnating
 powder doped with SrTiO3 by mixing it (including grains
 have a dimension 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 others
 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 small amount of water and gelling catalysts 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 a heat treatment, it is possible, for example, to heat at 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 boro-bismuthate or alumina silicate are obtained, respectively. Several successive layers in combinations 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.



   Another advantage relating to the use of solutions of alkoxides (this technique is commonly called Sol-gel) for the supply of the liquefiable phase necessary for sintering and subsequently providing the intergranular boundary layer is linked to improved control of the thickness of the barrier layer which can be exerted by leaving the grains of the conductive (or semi-conductive) powder in contact with a solution of the organometallic compounds for a longer or shorter time, so that the 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 temperature of
 sintering, resulting in a reduction in the tendency to get fat
 grains during sintering.



   As was briefly mentioned above, the materials
 intended to constitute the liquefiable phase of sintering aid may
 also be provided in powder form and simply mixed
 dipped in doped SrTiO3. To achieve this goal, we mix SrTiO3
 pulverulent with a selected amount of the sintering aid agent
 powder, after which this mixture is pressure molded in the presence
 a binder capable of being removed afterwards by decomposition
 (thus, a solution of a volatile or easy polymer can be used
 combustible, such as polyvinyl alcohol or polyvinyl
 butyral).



   It should be noted that the use in powder form of
 liquefiable sintering agents are not to be considered as
 a simple alternative to the use, for this purpose, of metallic alkoxides according to the above-mentioned Sol-gel technique. Indeed, a combination of the two techniques can be advantageous. Thus, for example, it is additionally possible to mix grains of SrTiO 3 already coated with Sol-gel with an additional quantity of powdered sintering aid agent before undertaking the actual sintering operation. 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, Bi2O3, H3BO3, Bacs3, P2Os and SrTiO3.

  The use of SrTiO3 as an additional component of the liquid sintering phase during the sintering of the semiconductor doped SrTiO2 grains is new and provides notable advantages with regard to the stabilization of the liquid phase as well as the permittivity and the Q factor. sintered ceramic. The preferred additional sintering aid compositions are as follows (in% by weight):
PbO 78% - GeO2 22%; PbO 45% - B2O3 5% - Pi2O3 50%;
PbO 25% -B2O3 20% -Bi203 20% -GeO2 5% By 10% -P2Os 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 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 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 doped SrTiO3.

  As the reducing atmosphere, dilutions of hydrogen or carbon monoxide can be used in an inert gas such as nitrogen, CO2 or a rare 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 have been prepared by mixing the powder coated with the liquefiable phase (in the presence or absence of an additional quantity of it in powder form) 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 about 10-20 mm in diameter and 1-3 mm in thickness, making sintering 1-6 h at a given temperature, fixing electrodes on either side of the disc and measuring its dielectric properties (capacitance and loss factor Q) by the usual means. The formula used is as follows (C measured in picofarads = 10-l2F):
 C = 0.0885g x S / e from which we calculated E (the permittivity, also designated by x), S being in cm2 and e in rm.

  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. Then, after sintering, either the discs are air-cooled or they are annealed as indicated in claim 1.



   It was also possible to implement the thermal air post-treatment of the invention by combining it with a diffusion post-treatment similar to that known in the prior art. Thus, under the applied conditions, the sintered ceramic is subjected to an annealing treatment and, at the same time, a vitreous material in the molten state is diffused inside the material, so as to further improve its dielectric properties. 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, pulverulent vitrifiable oxides were kneaded with a metallic paste or 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 of the ceramic, thus providing the desired electrodes.



   Some of the compositions which are preferred 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% - SrTiO2 2% - Ag paste 60% (d) PbO 18% -Bi2O2 20% -P2O2 2% - Ag paste 60% (e) PbO 78.05% -GeO2 21.95% (f) PbO76% -GeO2 19% -PaO 5% (gl) PbO (g2) PbO 60% -PbF 40% (h) PbO 20% - Pi2O3 20% - B2O3 40% - CuO 20% ( i) PbO 66% - GeO2 12% -Bi203 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 (put) 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 P2O3 / 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 TiO2 (grain size 0.75 km) and 1.01 g of Nb2Os (0, 7 mole%). The mixture was heated for 2 hours at 1100 "C in air (platinum crucible) and, after cooling, it was further heated for 4 hours at 1480" C in an H2 / Ar mixture containing 10% H2 (V / V) so as to ensure a reducing effect and to cause the appearance of 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 0.5-5 11 approximately.

  The powder was then wrung out and dried. Its resistivity, measured by the usual means, was 4.88 Q cm. 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-ethylhexanoate11, 6 g of acetylacetone, 13 g of ethanol and 10 mg of HCl (in the form of an 0.1N ethanolic solution). After standing for about 1/2 h, the powder was drained, dried for 16 h at 90 "C, then heated for 1 h in air at 550" C (4 "C / min This treatment was repeated 4 times in succession.

  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 / tbutanol, it was dried, passed through a 300 µm mesh screen and pressure molded into 1 mm green discs (3T / cm2). Was heated in air, 1 h at 550 "C to destroy the binder (0.5" C / min).



   The green disks were then sintered under the conditions indicated in Table I and the dielectric properties were measured as follows:
 After sintering, Pt electrodes were applied to the discs by spraying according to the usual means and the properties of the dielectric were measured at 1.5 kHz using a HEWLETT-PACKARD HP-4192-AIF electrotest apparatus provided with a 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.



   Table I
EMI4.1


 <tb> <SEP> Sintering <SEP> Properties <SEP> dielectrics
 <tb> Echan
 <tb> furrow <SEP>, atm, <SEP> heating <SEP> temp. # <SEP> Q <SEP> R
 <tb> <SEP> (c / min) <SEP> (c) <SEP> (min) <SEP> (M @)
 <tb> I'-C <SEP> N2 <SEP> 10 <SEP> 1250 <SEP> 180 <SEP> 6692 <SEP> 2.8 <SEP> 10
 <tb> I'-E <SEP> N2 <SEP> 10 <SEP> 1100 <SEP> 240 <SEP> 840 <SEP> 000 <SEP> 0.6 <SEP> 2.4
 <tb> I'-F <SEP> N2 <SEP> 7 <SEP> 1350 <SEP> 60 <SEP> 240 <SEP> 000 <SEP> 0.4 <SEP> 22
 <tb> I'-G <SEP> N2 <SEP> 8 <SEP> 1060 <SEP> 120 <SEP> 4857 <SEP> 2.9 <SEP> 65
 <tb>
 The I'E sample was air annealed for varying lengths of time at various temperatures. The results, recorded in Table II, indicate that the annealing decreases the permittivity but increases the Q factor.



   Table II
EMI4.2


 <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
 was about 1 to 100 nm.



  Example 2
 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 TiO2 and 11.72 g (0.044 mole) of Nb2O0. As the carrier phase, water was used. The powder cake was air dried, then heated for 2 h at 1100 "C in air in an alumina crucible (heat-up rate, 400" C / h). A reduction was then carried out by heating 2-4 h at 1450-1480 "C under a mixture of 10:90 (V / V) of hydrogen / nitrogen. The heating rate was approximately 300 to 420" C / h. Then the reduced product was ground in a polyethylene mill in the presence of WC / Co beads to a particle size of 0.5 to 5 μm. As the milling liquid, a 75:25 mixture of petroleum ether / t-butanol was used.

 

   The ground doped SrTiO2 thus obtained was washed with acetone and, after having dried it, it was brought into contact, approximately 30 min, in a glove box, with a chosen solution of the Sol-gel type. For this, the following solutions were used, called HM9SIA and PGI.



   HM9SIA (abbreviated HM): V2 h was mixed and heated at 70 "C 41.6 g of Si (EtO) 4.30 g of n-butanol, 7.9 g of dibutoxide acetylacetate aluminum chelate 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 h was heated again at 50 ° 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 0.1N solution in ethanol); allowed to cool and used at room temperature.



   After bringing the crushed 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. then densified the powder by heating it for 1 hour 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 / 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 under 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 the one hand, 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, to the green disc, paints or pastes to be metallized, for example containing platinum, copper. silver and other poorly oxidizable metals. Table III below summarizes certain operating data and the corresponding results obtained, in terms of dielectric properties. The table indicates the implementation of the thermal post-treatments of the invention, such as cooling in the presence of air after sintering (C). Such treatment (analogous to annealing) is indicated by the cooling rate in "C / h. The absence of such treatment corresponds to the control tests.

  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
EMI5.1


 <tb> <SEP> Solution <SEP> Sintering <SEP> Properties
 <tb> Trial <SEP> Sol-gel <SEP> <SEP> Sol-gel
 <tb> <SEP> N " <SEP> number <SEP> from <SEP> temp. <SEP> time <SEP> post-processing <SEP> E <SEP> Q
 <tb> <SEP> layers <SEP> ("C) <SEP> (h) <SEP> ("C / h) <SEP>
 <tb> H4 (1) <SEP> OHM, <SEP> 5PAGI <SEP> 1230 <SEP> 2 <SEP> - <SEP> <SEP> 7760 <SEP> 10.5
 <tb> H4 (3) <SEP> OHM, <SEP> 5PSI <SEP> 1263 <SEP> 2 <SEP> - <SEP> <SEP> 5000 <SEP> 14.3
 <tb> H4 (4) <SEP> OHM,

    <SEP> SPI <SEP> 1248 <SEP> 3 <SEP> 180 <SEP> 9000 <SEP> 96.2
 <tb> HS (1) <SEP> SPGI <SEP> 1230 <SEP> 2 <SEP> - <SEP> <SEP> 7000 <SEP> 31
 <tb> H5 (2) <SEP> 5PGI <SEP> 1248 <SEP> 2 <SEP> - <SEP> <SEP> 7500 <SEP> 9.7
 <tb> H5 (3) <SEP> 5PGI <SEP> 1263 <SEP> 2 <SEP> - <SEP> <SEP> 5200 <SEP> 17.0
 <tb> H5 (4) <SEP> 5PGI <SEP> 1248 <SEP> 3 <SEP> 180 <SEP> 6000 <SEP> 99
 <tb>
 Air cooling increases the Q factor without adversely affecting the permittivity. On the other hand, 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 the consequence of a somewhat greater combination of the values E and Q.



  Example 3
 The materials of Example 2 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 with Solgel, this operation being carried out as follows:
 1000 "C (glass melting temperature) in an alumina crucible a PbO-GeO2 mixture 78:22 by weight (called 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 2 (polyethylene mill, WC / Co balls) until 'at a grain size of about 1-5 µm.



   The SiO2 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 3 g of solids. The grout was then wrung out and treated and sintered into shapes as in Example 2.



   The results of the measurements carried out on the 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 2; in other cases (A), it is annealing at the temperature and for the time indicated.



  Table IV
EMI5.2


 <tb> <SEP> Sintering <SEP> properties
 <tb> Trial <SEP> N <SEP> Solution <SEP> Sol-gel <SEP> Phase <SEP> temp. <SEP> time <SEP> post-processing <SEP> @ <SEP> Q
 <tb> <SEP> 1 layer <SEP> liquefiable <SEP> (C) <SEP> (h) <SEP> refr. <SEP> C ( <SEP> C / h) <SEP> or
 <tb> <SEP> (% <SEP> in <SEP> weight) <SEP> annealed <SEP> A ( <SEP> C,

    <SEP> h)
 <tb> <SEP> H1 (2) <SEP> lHM <SEP> PGO <SEP> 4% <SEP> 1170 <SEP> 2
 <tb> <SEP> 1270 <SEP> 2 <SEP> - <SEP> 3400 <SEP> 16.2
 <tb> <SEP> Ho (3) <SEP> îHM <SEP> PGO <SEP> 4% <SEP> 1263 <SEP> 2 <SEP> A (1100.1) <SEP> 32000 <SEP> 5.3
 <tb> <SEP> H3 (l) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1230 <SEP> 2 <SEP> - <SEP> <SEP> 5100 <SEP> 18.3
 <tb> Tablear IV (continued)
EMI6.1


 <tb> <SEP> Sintering <SEP> Properties
 <tb> Solution <SEP> Sol-gel <SEP> Phase
 <tb> Trial <SEP> N0 <SEP> 1 <SEP> layer <SEP> liquefiable <SEP>

   temp. <SEP> time <SEP> post-processing
 <tb> <SEP> ( <SEP> in <SEP> weight) <SEP> (O <SEP> C) <SEP> (h) <SEP> refr. <SEP> C (0 <SEP> C / h) <SEP> or <SEP> E <SEP> Q
 <tb> <SEP> annealed <SEP> A (0 <SEP> C,

    <SEP> h)
 <tb> <SEP> H3 (2) <SEP> 1HM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> 2 <SEP> - <SEP> 10800 <SEP> 1,2
 <tb> <SEP> H3 (3) <SEP> 1HM <SEP> PGO <SEP> 5% <SEP> 1263 <SEP> 2 <SEP> - <SEP> <SEP> 3700 <SEP> 10.8
 <tb> <SEP> H3 (4) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1263 <SEP> 2 <SEP> A (1100.1) <SEP> 9600 <SEP> 13
 <tb> <SEP> H3 (5) <SEP> îHM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> 3 <SEP> C (180) <SEP> 11600 <SEP> 100
 <tb> <SEP> H3 (6) <SEP> 1HM <SEP> PGO <SEP> 5% <SEP> 1305 <SEP> 1
 <tb> <SEP> no <SEP> from <SEP> H2 <SEP> - <SEP> <SEP> 5400 <SEP> 11
 <tb> <SEP> H3 (7) <SEP> lHM <SEP> PGO <SEP> 5% <SEP> 1248 <SEP> 3 <SEP> C (180) <SEP> 10650 <SEP> 116
 <tb>
Example 4
 This example is

   intended to illustrate the application, after sintering, of an air post-treatment combined with the diffusion of a vitreous composition in the molten state inside the ceramic. The doped reduced strontium titanate is the same as in Examples 2 and 3 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 semiconductor doped SrTiO2 grains were prepared by calcining (1100 C, 2 h) a mixture of 1000 g of SrCO2, 541 g of TiO2 and 12.69 g of Nb2O5 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 2) and, after the usual treatments, the powder was mixed with 5% (by weight) of PGO (see Example 3); 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
EMI6.2


 <tb> <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100 <SEP> KHz
 <tb> Trial <SEP> N <SEP> temp. <SEP> time <SEP> composition <SEP> temp. <SEP> time <SEP> gain <SEP> in <SEP> # <SEP> Q <SEP> # (# cm)
 <tb> <SEP> (C) <SEP> (h) <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 (11) <SEP> 1206 <SEP> 5 <SEP> gl <SEP> 920 <SEP> 3 <SEP> 3.5 <SEP> 6450 <SEP> 94 <SEP> 21x109 <SEP>
 <tb> H3 (12) <SEP> 1180 <SEP> 6 <SEP> 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> 14x109
 <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 with little hydrogen, that is to say less than 0.4% in nitrogen (V / V). The operating parameters, the diffusion conditions and the results are recorded in Table VB below:
Table VB
EMI6.3


 <tb> <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> to <SEP> 100 <SEP> kHz
 <tb> Trial <SEP> N <SEP> temp. <SEP> time <SEP> composition <SEP> temp.

   <SEP> time <SEP> gain <SEP> in <SEP> # <SEP> <SEP> Q <SEP> # (#. Cm)
 <tb> <SEP> (C) <SEP> (h) <SEP> (C) <SEP> (h) <SEP> weight (%) <SEP> DC <SEP>
 <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> 17x <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> 10S <SEP>
 <tb> H34 (4) <SEP> 1284 <SEP> 4 <SEP> g2 <SEP> 910 <SEP> 3 <SEP> 5.3 <SEP> 5650 <SEP> 133 <SEP> 11x109 <SEP>
 <tb> H34 (5) <SEP> 1284 <SEP> 4 <SEP> f <SEP> 940 <SEP> 3 <SEP> 19.2 <SEP> 3450 <SEP> 31 <SEP> 36x <SEP>

     107 <SEP>
 <tb> <SEP> H34 (6) <SEP> 1284 <SEP> 2 <SEP> g2 <SEP> 930 <SEP> 3 <SEP> 3.8 <SEP> 11200 <SEP> 30 <SEP> 2x108 <SEP>
 <tb> <SEP> 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> <SEP> H34 (8) <SEP> 1256 <SEP> 4 <SEP> f <SEP> 1200 <SEP> 2 <SEP> 7.7 <SEP> 2800 <SEP> 16 <SEP> 28x108 <SEP>
 <tb> <SEP> H34 (9) <SEP> 1256 <SEP> 4 <SEP> e <SEP> 1200 <SEP> 2 <SEP> - <SEP> 000 <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 nitrogen containing a quantity of hydrogen not exceeding 0.4% (V / V). The results are shown in Table VC below:
Table VC
EMI7.1


 <tb> <SEP> Sintering <SEP> Diffusion <SEP> Properties <SEP> a <SEP> 100 <SEP> kHz
 <tb> Trial <SEP> N
 <tb> <SEP> temp. <SEP> time <SEP> composition <SEP> temp.

   <SEP> time <SEP> gain <SEP> in <SEP> E <SEP> Q <SEP> # (#. Cm)
 <tb> H33 (1) <SEP> 1362 <SEP> 2 <SEP> f <SEP> 1100 <SEP> 1.5 <SEP> 17.4 <SEP> 17400 <SEP> 22.3 <SEP> 125 <SEP> x <SEP> 108
 <tb> H33 (10) <SEP> 1284 <SEP> 4 <SEP> f <SEP> 940 <SEP> 3 <SEP> 21.6 <SEP> 5100 <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 <SEP>
 <tb>
Example 5
 Strontium titanate doped with 0.7 mol% of niobium as prepared as indicated in Example 2 was mixed, as described in Example 3, with liquefiable sintering aid materials, no preliminary coating resulting from the Sol-gel technique not being applied.

  The sintering aid compositions used were the following (by weight): PbO 78% -GeO2 22% (PGO); PbO 45%
   B203 5% - Bizou 50% (BPBO); PbO 25% - B203 20% - GeO2 5% Bizou 20% - BaO 10% - P2Os 20% (ZPhBa). In some cases, the compositions also contained 5% by weight (based on the total of the composition) of SrTiO2.



   By pressure, discs were formed as described in the previous examples and these were subjected to sintering under nitrogen or to an H 2 / N 2 mixture as described above. Then, the sintered samples were subjected to an annealing (A) or air cooling (C) treatment according to the invention under the conditions summarized in Table VI which follows. The operating conditions, such as nature and quantity of the liquefiable phase, the sintering conditions and the dielectric properties are also shown in the table. The control samples were not subjected to any air post-treatment.



  Table VI
EMI7.2


 <tb> <SEP> Fnttage <SEP> Properties <SEP>
 <tb> Trial <SEP> N <SEP> Agent <SEP> from <SEP> sintering <SEP> dielectrics
 <tb> <SEP> (%) <SEP> temp. <SEP> time <SEP> post-processing
 <tb> <SEP> (C) <SEP> (h) <SEP> C (C / h);

   <SEP> A (C, h); A (C, h) <SEP> # <SEP> Q
 <tb> <SEP> H7 (2) <SEP> PGO <SEP> (5) <SEP> 1263 <SEP> 2 <SEP> - <SEP> 11500 <SEP> 29.3
 <tb> <SEP> H7 (3) <SEP> PGO (5) <SEP> 1263 <SEP> 2 <SEP> A (1100, <SEP> 1) <SEP> 4300 <SEP> 62.5
 <tb> <SEP> H7 (4) <SEP> PGO <SEP> (5) <SEP> 1263 <SEP> 2 <SEP> C (180) <SEP> 4700 <SEP> 112
 <tb> <SEP> H8 (2) <SEP> 3PGO: 2BPBO <SEP> (5) <SEP> 1263 <SEP> 2 <SEP> - <SEP> 13400 <SEP> 37.3
 <tb> <SEP> H8 (3) <SEP> 3PGO: 2BPBO (5) <SEP> 1263 <SEP> 2 <SEP> A (1100, <SEP> 1) <SEP> 5640 <SEP> 66.5
 <tb> <SEP> H8 (4) <SEP> 3PGO: 2BPBO <SEP> (5) <SEP> 1248 <SEP> 3 <SEP> A (1100, <SEP> 1) <SEP> 3300 <SEP> 80
 <tb> <SEP> H8 (5) <SEP> 3PGO:

  : 2BPBO <SEP> (5) <SEP> 1248 <SEP> 3 <SEP> C (180) <SEP> 10300 <SEP> 138
 <tb> <SEP> H11 (2) <SEP> PGO <SEP> (5) <SEP> 1263 <SEP> 2 <SEP> - <SEP> 13300 <SEP> 3.9
 <tb> <SEP> H11 (3) <SEP> PGO <SEP> (5) <SEP> 1263 <SEP> 2 <SEP> A (1100, <SEP> 1) <SEP> 3900 <SEP> 60
 <tb> <SEP> H12 (1) <SEP> ZPhBa (5) <SEP> 1248 <SEP> 3 <SEP> A (1100, <SEP> 1) <SEP> 3200 <SEP> 94
 <tb> H12 (2) <SEP> ZPhBa <SEP> (5) <SEP> 1248 <SEP> 3 <SEP> C (180) <SEP> 6400 <SEP> 150
 <tb> H14 (2) <SEP> ZPhBa <SEP> (15) <SEP> 1100 <SEP> 1 <SEP> C (180) <SEP> 930 <SEP> 5.2
 <tb> H14 (3) <SEP> ZPhBa (15) <SEP> 1100 <SEP> 1 <SEP> A (1000,

    <SEP> 1) <SEP> 5500 <SEP> 32
 <tb> H15 (1) <SEP> 95PGO / 5SrTiO3 <SEP> (5) <SEP> 1248 <SEP> 3 <SEP> C (180) <SEP> 3650 <SEP> 98
 <tb> <SEP> H16 (1) <SEP> BPBO / SrTiO3 <SEP> (5) <SEP> 1248 <SEP> 3 <SEP> C (180) <SEP> 9900 <SEP> 94
 <tb> H17 (1) <SEP> ZPhBa / SrTiO3 <SEP> (5) <SEP> 1248 <SEP> 3 <SEP> C (180) <SEP> 4500 <SEP> 130
 <tb> H19 (1) <SEP> BPBO / SrTiO3 <SEP> (15) <SEP> 970 <SEP> 3 <SEP> C (from <SEP> 840 <SEP> 1650 <SEP> 39
 <tb>


    

Claims (15)

 CLAIMS  1. Method for manufacturing ceramics with an internal granular phase limited by an insulating barrier layer of high permittivity suitable for large-capacity multilayer capacitors, according to which the following steps are carried out:  (a) powdered semiconductor doped strontium titanate is prepared, the grains of which have an average size of 0.5 to 5 μm;  (b) a sintering aid phase in the solid state is mixed with said grains, this phase causing the formation by baking of said barrier layer by sintering in liquid phase in a neutral or slightly reducing atmosphere;  (c) a slurry of this mixture is formed with an organic binder which is thermally decomposable and this slurry is molded under pressure in the form of green articles intended to constitute dielectrics for capacitors; ;  (d) these green articles are subjected to cooking under an inert atmosphere or in a gas containing hydrogen or carbon monoxide, the reducing power of which is sufficient to preserve the semiconductor properties of the grains of SrTiO3, said cooking causing sintering and densifying said articles in ceramic bodies having dielectric properties;  (e) these sintered bodies are cooled to ambient temperature; this process being characterized by the fact that said sintered body is subjected to a thermal post-treatment with air, this post-treatment consisting either of carrying out step (e) in air according to a predetermined program of variation of temperature, or, after step (e), in an additional annealing treatment.  2. Method according to claim 1, characterized in that step (b) is carried out by the Sol-gel technique, that is to say that the grains of SrTiO3 doped with solutions are impregnated metal alkoxides. these alkoxides providing, by gelation, a coating of metalloxane on these grains, the powder thus coated is dried and it is heat treated to transform the coating into said solid phase of sintering aid, the latter containing metal oxide compounds.  3. Method according to claim 2, characterized in that said alkoxides are chosen from Pb, Ge, Bi and B alkoxides, the metal oxide compounds of said phase consisting of lead germanium of lead-boron bismuthate bismuthate .  4. Method according to claim 2, characterized in that in step (b) one begins by coating the SrTiO3 grains with an alumina silicate film by means of gelling solutions of silicon alkoxides and aluminum.  5. Method according to claim 1, characterized in that step (b) is carried out by mechanically mixing the grains of doped SrTiOa with one or more powder sintering aid compositions, these compositions containing oxides of elements chosen from Pb, Ge, Bi, P and Ba.  6. Method according to claim 5, characterized in that the doped SrTiO3 used comprises an initial coating of metal oxide obtained according to the method of claim 2.  7. Method according to claim 1, characterized in that by carrying out step (e), the sintered body is cooled in air from the sintering temperature to room temperature at the rate of 50 to 250 C / h .  8. Method according to claim 1, characterized in that in step (e) the sintered body is first cooled in a sintering atmosphere from the sintering temperature to a predetermined temperature and thereafter at room temperature, in air, at a rate of 50 to 250 C! h.  9. Method according to claim 8, characterized in that said predetermined temperature is from 800 to 1100 "C and that the cooling rate is 150-200" C / h.  10. Method according to claim 1, characterized in that in the case of an annealing treatment, the sintered body is heated in air or under oxygen at a temperature of 800 to 1200 "C for 0.5 to 5 h.  11. Method according to claim 10, characterized in that before heating the sintered body, a vitrifying composition is applied to the surface of the latter, 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.  12. Method according to claim 11, characterized in that said vitrifiable composition consists of a grout or paste of powdered mineral matter applied to the surface of the sintered body.    13. The method of claim 12, characterized in that said vitrifiable composition comprises at least one of the following compounds: PbO, GeO2, Bd203, PbF2, CuO, SrTiO2.  14. The method of claim 12, characterized in that said paste further contains a metal in the divided state, this metal forming a metal film on the ceramic surface during diffusion and being subsequently usable as a capacitor electrode.  15. The method of claim 1, characterized in that the reducing atmosphere in step (d) consists of a mixture of hydrogen and a rare gas or nitrogen, or monoxide and dioxide carbon, and that this step (d) is carried out at temperatures of approximately 950 to 1310 C.