EP1529022A2

EP1529022A2 - High-density ceramics and method for the production thereof

Info

Publication number: EP1529022A2
Application number: EP03790724A
Authority: EP
Inventors: Christian Pithan; Rainer Waser; Franz-Hubert Haegel
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2002-08-14
Filing date: 2003-08-14
Publication date: 2005-05-11
Also published as: WO2004020363A2; DE10237915A1; WO2004020363A3

Abstract

Disclosed is a method for producing high-density ceramics, in which oxidic nanoparticles are first produced by hydrolyzing water-sensitive compounds with microemulsifiers. The resulting particles which have the desired stoichiometric composition are processed to high-density ceramics at very low temperatures by means of pressure-assisted methods. Ceramics having a dense grain size distribution are created from the initial powders that are produced via microemulsifiers due to the dense particle size distribution. The growth rate of the grains during the sintering process is regular as a result of the uniformity of the primary particles. Ceramics that are produced according to the inventive method are also provided with smaller leakage currents due to the temperature characteristics of the dielectric material properties, which are controlled by the grain size, because the grains grow only slightly during sintering, keeping the proportion of electrically blocking grain boundaries high.

Description

description

High density ceramics and processes for

Making the same

The invention relates to ceramics, in particular high-density ceramics with advantageous mechanical, dielectric, piezoelectric or magnetic properties. The invention further relates to a method for producing these ceramics, in particular a method for producing oxidic nanoparticles as the starting substance for the high-density ceramics.

State of the art In contrast to construction ceramics, in which a low weight and good chemical, mechanical and thermal resistance are important, the so-called functional ceramics are used due to different material properties. These include, for example, temperature-dependent electron (insulators, semiconductors, superconductors) or ion conduction (sensors, membranes, fuel cells), magnetism (permanent magnets, data storage), dielectric, piezo or pyroelectric behavior and catalytic activity.

The area of application of the functional ceramics is therefore to be found preferably in the field of microelectronics (insulators, semiconductors, sensors, actuators) and conventional electronics (resistors, capacitors, thermistors and varistors), as well as electrical engineering (HT heating conductors). The number of suitable materials as functional ceramics is constantly increasing. The relevant material properties of these ceramics are causally related to the atomic structure, especially the crystal structure.

For example, the barium titanate that is tetragonal at room temperature and crystallized in a perovskite structure forms a ferro- and piezoelectric phase. If the Curie temperature is exceeded, a change of modification to a cubic, paraelectric structure takes place. The shift in atomic positions associated with the tetragonal structure leads to the formation of a permanent dipole moment and thus to a high dielectric constant.

The Curie temperature can be set by targeted doping, above which there is also a strong change in the specific resistance.

Synthesis of the starting powder

When manufacturing ceramics from powdery solid raw materials, many parameters have to be observed regularly. During shaping, the processed raw materials are first compressed. During the sintering process, the second solidification takes place due to crystallization and grain growth processes, which can often be coupled with reactive reactions.

The properties of the finished ceramic depend to a large extent on the phase composition of the solid, the crystal size and the porosity. Ceramics are manufactured from an inhomogeneous material Mix of solid raw materials below the melting point. Therefore, transport processes and diffusion paths of the ions or atoms are of crucial importance for the formation of homogeneous and high-density ceramics.

The influence of the particle size becomes clear when the sintering temperature is significantly reduced. The improved mechanical stability of ceramics made of nanoparticles is based on an improved homogeneity and a controlled particle size distribution in the green bodies. The production of only weakly agglomerated starting powders leads to the production of dense, uniform and fine-grained ceramic microstructures, which have a significantly reduced sintering temperature, via also only weakly agglomerated ceramic powders. Studies on the production of non-agglomerated, ultrafine powders for the systems yttrium, titanium and yttrium-stabilized zirconium oxide from [1] are known. Examination of the starting powders in a scanning electron microscope reveals an average particle size of approximately 20 nm for the titanium dioxide and 10 to 20 nm for the yttrium-stabilized zirconium oxide.

Processes for producing nanocrystalline mixed oxide ceramic powders by means of a microemulsion-based sol-gel process are described in the literature. [2] discloses the production of nanocrystalline ceramic powders using two process routes. On the one hand the generation of nano-crystalline BaTi0 ₃ by hydrolysis of reactive precursor in water-filled inverse micelles [3] and on the other hand the production of NH ₄ MnF ₃ by an precipitation reaction of various metal salts in inverse micelles.

The synthesis of nanoparticles from alkoxides with microemulsions using the method described above has already been described in [4] by Beck, Härtl and Hempelmann. The process is similar to a sol-gel process with regard to the starting compounds and is therefore also referred to as such by the authors. However, there are fundamental differences with regard to the processes taking place during the synthesis and the products obtained. In particular, the narrow droplet size distribution in the microemulsion gives such a narrow distribution of the particle sizes that cannot be obtained in a sol-gel process in homogeneous solvent mixtures. The size distribution, characterized by the diameter d, of the particles produced was based on a logarithmic normal distribution

(ln <- / - lnμ) ²

G _LN (d) = exp

XΪπ d lnσ 2 (lnσ)

customized. The particles had mean sizes of 3.0 to 15.9 nm for different compositions of the microemulsion. The width of the particle size distribution σ was 1.1 to 1.9 nm. In the worst case, the smallest particles with 3.0 nm correspond to a share of <5% for particles with a diameter greater than 9.0 nm, the Three times the mean. The corresponding proportions are much lower for the larger particles. Hempelmann et. al. Ti0 ₂ , Zr0 ₂ , SrTi0 ₃ , BaZr0 ₃ , SrZr0 ₃ , CaTi0 ₃ , CaZr0 ₃ , Ba _x Srι- _x Ti0 ₃ , BaY _x Zrι- _x 0 ₃ , MgAl ₂ 0 _4, CoAl ₂ 0 _, NiAl ₂ 0 _/ CuAl ₂ 0 ₄ , BaZr _o , BaZro, 85l ⁿ o, 1502.925 and BaZro, 8sCao, 1502.925 produced via this synthesis route and described in several publications.

The synthesis of potassium niobate from the alkoxides via a sol-gel process is known from [5]. Despite the stoichiometric double alkoxide used by the authors, pure KNb0 ₃ phases could not always be obtained. Rather, the properties of the product were heavily dependent on the solvent. As a rule, the products were deficient in potassium.

The hot pressing and sinter forging of materials with particles <10 nm (SiC _x N _y and ZrO _x C _y ) is e.g. B. from Kear, Chag, Skandan and Hahn in US 5,514,350. However, the particle separation takes place from the gas phase and therefore only achieves low particle concentrations. The particle size distribution is also wider than for microemulsion synthesis. Not all materials can be easily separated with the correct stoichiometry via the gas phase.

Manufacture of ceramics

Furthermore, various methods for shaping the ceramic starting powder z. B. dry pressing, extruding, injection molding, film casting or

Known slip casting. This is regularly followed by a thermal treatment with which the final structure formation in the ceramic is carried out. The pro The process of material compaction and consolidation is completed. For the expulsion of organic additives, a binder burnout is first carried out (RT up to max. 500 ° C), whereby the moisture is also removed. Only then does the actual sintering process typically take place at temperatures up to 1700 ° C.

In addition to the various sintering methods under atmospheric conditions with or without protective gas, the literature also describes pressure-assisted consolidation (hot pressing, sintering, etc.) of materials that are produced in the liquid phase.

The pressure-assisted consolidation of loosely aggregated ceramic nanopowders with a metastable structure is described by Kear, Liao and Mayo in US Pat. No. 6,395,214. The pressures used are very high. They are 3-5.5 GPa. The temperature is given as 0.2 to 0.6 times the absolute melting temperature. The powders used have small particles of 10-20 nm, but no narrow particle size distribution (FIG. 2 of the cited patent).

The pressure-supported consolidation of materials that were generated in the liquid phase was described in [1] for yttrium-stabilized zirconium dioxide with particle sizes of 10-20 nm. The processes mentioned in the publication for the synthesis, however, do not guarantee a narrow particle size distribution.

The authors provide for particles that are larger than 1 micron, ^"a proportion of from 85-90%. In the same publication are dependent on the exact Synthesebedin- Y ₂ 0 ₃ powder with different sintering properties. Powders with sharp-edged, strongly agglomerated particles have proven unsuitable for the production of dense ceramics, while powders with weakly agglomerated particles can be sintered extremely well.

applications

Many properties of ceramics depend crucially on the grain size and the grain size distribution. This applies not only to the mechanical, but also to other physical properties as used in certain functional ceramics. Ceramics are mainly used in the fields of electronics, optics, mechanical engineering and medicine. In the field of electro-ceramic materials, ceramics with magnetic, ferroelectric, dielectric, piezoelectric, pyroelectric, photoelectrochemical, semiconducting, ion-conducting, superconducting, electro-optical properties have a wide variety of applications.

Ferroelectrics exhibit a structural phase transition from a low-temperature ferroelectric phase to a high-temperature paraelectric phase, which is generally associated with a very strong dependence of the dielectric constants of the material on the temperature. Around the range of the Curie temperature T _c , which characterizes the transformation point, both the dielectric and the piezoelectric coefficients have maximum values which drop drastically for both lower and higher temperatures. Such a pronounced temperature behavior is indispensable in many technological applications. wishes, because in the use of the material a possible warming caused by the operation affects the change of the characteristic values of the corresponding component. It is known that the grain size of electroceramic materials has a very strong effect on the ferroelectric phase transition. As a rule, the destabilization of ferrodelect-

I polarized phase observed. This manifests itself among other things in a shift in the Curie temperature to lower values and in a broadening of the characteristic maximum in the temperature dependence of the dielectric constant (Curie peak). In practice, materials are positwerkstoffen having such a temperature characteristic already in the form of com- that a so-called core-shell ^'.

Have structure used. Classic application examples are multi-layer capacitors (eg of type X7R), in which a particularly low temperature dependency of the dielectric constant is required. With increasing trend towards miniaturization and

Increasing the volume efficiency of such components decreases the layer thickness of the dielectric layers more and more. As a result, the field load in the dielectric ceramic increases with the operating voltage remaining the same. This increased field load can

Cause material failure due to degradation of the insulation resistance. In principle, these failure mechanisms occur in all dielectric materials. They are induced by the applied DC voltage. Accelerated failure occurs especially at elevated operating temperatures. Grain boundaries have an inhibiting effect on the degradation of the resistance, since they act as Barriers act and delay the onset of ionic leakage [6].

OBJECT AND SOLUTION The object of the invention is to create high-density ceramics with particularly advantageous properties with regard to strength, density and mechanical, dielectric, piezoelectric or magnetic properties. Furthermore, it is the task of the invention to provide a production method for this type of ceramic with the corresponding ceramic starting powder in the form of nanoparticles.

The objects of the invention are achieved by a production method for a high-density ceramic made of nanoparticles with all the features of the main claim, and by high-density ceramics with all the features of the secondary claim. Further advantageous refinements of the method and the ceramic can be found in the claims which refer back to them.

Subject of the invention

The invention relates to a method for producing high-density ceramics from nanoparticles, and materials in the form of nanoparticles and high-density ceramics.

The process produces oxidic nanoparticles by hydrolysing water-sensitive compounds with microemulsions. The particles of the desired stoichiometric composition generated in this way can already be used with the aid of pressure-assisted processes very low temperatures can be processed into high-density ceramics.

The ceramics are supported by pressure-supported consolidation, e.g. B. by gas pressure sintering, hot pressing or sintering from powders, the

Primary particles have an average size <100 nm, preferably <50 nm and particularly preferably <10 nm. The primary particles are by hydrolysis of alkoxides or a mixture of alkoxides and other easily hydrolyzable compounds, such as. B. halides, oxalates or acetates, with microemulsions. For the synthesis, the compound to be hydrolyzed or the compounds to be hydrolyzed are initially introduced, at least in part, in a solvent which can be miscible or immiscible with water and hydrolyzed by adding a microemulsion. This produces oxide or hydroxide or mixed oxide-hydroxide nanoparticles with a very narrow size distribution, with less than 10%, preferably less than 5% and particularly preferably less than 2% of the particles

Size larger than three times the medium size.

This microemulsion contains at least the three components, water, a water-immiscible solvent and a surfactant. Technical surfactants are usually already multi-component mixtures. In addition, other components can be added, such as. B. alcohols as cosurfactants or one or more of the reactants or buffers. The surfactants can be non-ionic, anionic or cationic. Nonionic surfactants are preferably used, since they can be removed without residue by thermal decomposition. For the hydrolysis, preference is given to using compounds which are implemented very quickly, such as, for. B. alkoxides (eg: barium iso propoxide, titanium iso propoxide, sodium methoxide, potassium ethoxide, niobium ethoxide, iron ethoxide).

The synthesis is not limited to the elements and compounds mentioned. In principle, any hydrolyzable compound which forms the corresponding oxide, hydroxide or an oxide hydroxide is suitable. The oxides can be obtained from hydroxides and oxide hydroxides by calcining.

If mixed oxides of several elements are to be produced, it is advantageous to present the components together to obtain a stoichiometrically composed product. It when a stoichiometric compound is presented, such as. B. in the case of BaTi0 ₃ , where the stoichiometric 1: 1 composite mixed barium titanium iso-propoxide is formed, which is then hydrolyzed with a microemulsion. In this way, the stoichiometric composition of the oxides can be set very precisely and easily if parts of the compound or of the product do not decompose during and after the formation of the particles. The reduction in the polarity of the solvent in the reaction mixture, which is brought about by the addition of the apolar phase of the microemulsion or by the use of a nonpolar solvent for the substance to be hydrolyzed, suppresses the decomposition of the product, as is particularly good in alcohol or Excess water soluble components of mixed oxides, hydroxides or oxide hydroxides can occur. In particular, especially when using alkali alkoxides, the solubility of the alkali ions in the form of their hydroxides can thus be greatly reduced and the preservation of the stoichiometry can be guaranteed regularly.

The oxides can be obtained in crystalline form during the synthesis. However, it is particularly advantageous for the subsequent compression to use the oxides in amorphous form. In order to keep the particles in amorphous form, it is necessary to increase the hydrolysis rate. This can be achieved, for example, by using particularly reactive hydrolyzable compounds, increasing the polarity of the water-immiscible solvent used for the microemulsion, by a high water content of the microemulsion, by increasing the temperature or by several of the measures mentioned.

The materials produced during the hydrolysis (oxides, hydroxides or oxide hydroxides) are then isolated and cleaned. The powders obtained generally consist of loose agglomerates of the primary particles. If necessary, they can first be freed of organic components by heating to over 500 ° C and then by a process for pressure-assisted consolidation, e.g. B. by gas pressure sintering, hot pressing or sinter forging at comparatively low temperatures and pressures to high-density ceramics. If the organic constituents can be largely separated off during cleaning, direct compression using pressure-assisted processes is also possible. If suitable surfactants are used, the surfactant from the manufacturing process itself, if appropriate after reducing the surfactant content in the raw powder by extraction with an organic solvent, can be a suitable binder for compacting the powder to form the green body. During the burnout, surfactants of the alkyl polyethoxylate type have proven to be advantageous over alkyl aryl ethoxylates.

The temperature required for compression drops considerably with decreasing particle size. It is therefore advantageous to use particles <100 nm, preferably <50 nm and particularly preferably <10 nm. The temperature required for the compression can be further reduced if the particles are in amorphous form. Even at comparatively low temperatures, this enables gentle conditions, the pressure-free production of dense ceramics with densities> 90%, preferably> 95%.

In contrast, e.g. B. KNaNb0 ₆ in the form of conventional powders in the range of> 100 nm can only be compacted (compressed) at temperatures just below the melting temperature of this compound at 1140 ° C under normal pressure to a maximum density of less than 94% of the theoretical density. The pressure-supported consolidation is preferably carried out at pressures <350 MPa, particularly preferably <200 MPa and ideally <30 MPa. The primary particles grow during sintering. Due to the narrow particle size distribution, ceramics with a narrow grain size distribution are created from the powders produced using microemulsions. Because of the uniformity of the primary particles, the growth rate of the grains is uniform during the sintering process. In contrast, in the case of non-uniform powders, sintering leads to a preferred growth of the large grains and thus to a greater increase in the mean particle sizes. This process plays an increasing role with increasing temperature.

Thanks to the pressure-supported sintering process (consolidation), oxidic powders can be compacted to high-density ceramics even at lower temperatures. In addition, the lower sintering temperature of the nanoparticles inhibits the growth of the primary particles. Therefore, the ceramics that can be produced by the combination of microemulsion synthesis and pressure-supported consolidation have particularly favorable properties, since they have a small average grain size and a uniform grain size distribution at high density. The densities are greater than or equal to 94%, preferably> 97%, particularly preferably> 99%. The mean grain size is <500 nm, preferably <100 nm and particularly preferably <50 nm. The ceramics also have a narrow grain size distribution, the proportion of grains with more than three times the mean grain size advantageously <20% and in particular <10 % is.

Ceramics that are produced in accordance with the method according to the invention have a through the Grain size-controlled temperature characteristics of the dielectric material properties additionally have smaller leakage currents, since the grain sizes grow only slightly during the sintering process and the proportion of electrically blocking grain boundaries thus remains high.

KNaNb ₂ 0 ₆ is a particularly advantageous piezoelectric material that has only a slightly smaller piezoelectric effect than that of Pb (Ti, Zr) 0 ₃ used in many areas. In medical technology, when it comes to implanting piezoceramic elements into the human body, materials containing Pb must be avoided due to their toxicity. Potassium sodium niobates could be an interesting biocompatible and toxicologically harmless alternative material. However, the compression of ceramic powders into compact ceramics is problematic. Due to the very low sintering activity, only very low relative densities below 94% can be set in conventional KNaNb ₂ 0 ₆ powders even at a temperature of 1100 ° C. A significant improvement in the sintering activity occurs in the ceramics which have been produced by the process according to the invention.

Due to the fineness of the starting powder and the use of pressure-supported consolidation, high-density ceramics can be achieved at a temperature of 1050 ° C and a pressure of 19 MPa. The ceramics obtained are translucent and thus almost have the theoretical density. At the high temperature compared to US Pat. No. 6,395,214, the uniformity of the particles has a particularly advantageous effect. because, despite the faster growth processes, the uniformity of the grains is largely retained and the grain growth is even.

Special description part

The subject matter of the invention is explained in more detail below on the basis of exemplary embodiments and figures, without the subject matter of the invention being restricted thereby.

Show it

Fig. 1: Nanocrystalline BaTi0 ₃ powder from microemulsion synthesis consisting of loosely aggregated primary particles (scanning electron microscope image).

Fig. 2: Nanocrystalline BaTi0 ₃ powder from microemulsion synthesis annealed at 800 ° C for one hour (scanning electron microscope image). Because of the uniformity of the primary particles, the growth rate of the grains is uniform during the sintering process.

3: X-ray diffraction diagrams of BaTi0 ₃ a) Comparison of the raw powder of the microemulsion synthesis with commercial powders b) after sintering the powder from the microemulsion synthesis at different temperatures Fig. 4: X-ray diffraction diagram of KNaNb ₂ 0 ₆ from the microemulsion synthesis

5: Comparison of the density of the BaTi0 ₃ powder sintered at different temperatures from the microemulsion synthesis with two commercial powders

A. Nanoparticle synthesis Al) Synthesis of crystalline BaTi0 ₃ using a microemulsion with an alkylaryl polyethoxylate Crystalline (pseudocubic) BaTi0 ₃ was prepared according to the procedure in [4]. For this, barium was dissolved in 2-propanol. After the stoichiometric amount of titanium tetra-iso-propoxide was added, the mixture was stirred for one hour. A microemulsion of cyclohexane, Tergitol NP 35, octanol and the stoichiometric amount of water was then added to the reaction mixture. The particles were isolated and purified by removing the solvent, boiling with acetone, filtration and Soxhlet extraction.

A.2) Synthesis of crystalline BaTi0 ₃ using a microemulsion with an alkyl polyethoxylate The synthesis was carried out in accordance with Al. In this case, however, the microemulsion used contained the components cyclohexane, Lutensol ON 110 (from BASF), octanol and water.

A.3) Synthesis of amorphous BaTi0 ₃

Amorphous BaTi0 ₃ was obtained when a microemulsion for the hydrolysis of the mixed barium-titanium-iso-propoxide was used with a polar oil. The composition of the microemulsion was Tergitol TMN 6 (Fluka) 4.970 g ethyl butyrate (Fluka) 12.456 g water 2.601 g

13.0 g of this microemulsion were added to a solution of barium titanium isopropoxide from 5.1 g barium, 10.8 g titanium tetra isopropoxide in 250 ml isopropanol. In contrast to example 1.1. Obtain a dry, fine powder as soon as the solvent has been removed; boiling with acetone to isolate the product was therefore not necessary. The powder was cleaned by Soxhlet extraction alone.

A.4) Synthesis of amorphous KNaNb ₂ 0 ₆

In the hydrolysis of a mixture of 0.373 g of potassium methoxide, 0.283 g of sodium ethoxide and 2.65 g of niobium pentethoxide in 50 ml of ethanol with 35.1 g of a microemulsion of the composition Tergitol NP 35 (Fluka) 10.426% cyclohexane 80.737%

1-octanol 6.051% water 2.787% KNaNb ₂ 0 _{5 was obtained} as an amorphous powder. The reaction mixture was filtered through a glass filter frit G3. The product was washed several times with cyclohexane. The amount of water used was 2.15 times the stoichiometric amount, that for the direct

Oxide formation was necessary. B. Characterization of the materials Bl) barium titanate

The X-ray diffraction diagrams (FIGS. 3a and 3b) show that the barium titanate is largely phase-pure. The material contained small amounts of BaC0 ₃ . 0.2% inorganic carbon was found in the raw powder. During sintering at low temperatures, the content of inorganic carbon increases to a maximum of 0.7% due to the oxidation of the organic components of the raw powder. At higher temperatures, the proportion is reduced to 0.05%. Compared to commercial powders, the peaks for the raw powder from the microemulsion synthesis are greatly broadened because of the small particle size (FIG. 3a). By sintering at different temperatures, ceramics with different particle sizes can be produced. With increasing sintering temperature, the grains grow and the peaks become narrower (Fig. 3b).

B.2) Potassium sodium niobate

The X-ray diffraction diagram of the potassium sodium niobate shows no foreign phases despite the addition of a stoichiometric amount of water after sintering of the amorphous material than at 1000 ° C. (FIG. 4).

C. Densification at normal pressure

Cl) Densification of crystalline BaTi0 ₃ from synthesis Al at normal pressure. FIG. 5 shows the comparison of the sintering curves for BaTi0 ₃ nanoparticles of 10 nm from the microemulsion synthesis according to 1.1. and two commercial powders with particle sizes of 50-70 nm or 2 μm. The data show that the powder from microemulsion synthesis in the range of 900-1150 ° C already achieves higher densities at lower temperatures than the comparison powder.

D. Pressure assisted compaction

Dl) densification of crystalline BaTi0 ₃ from synthesis A.2 under high pressure

The surfactant content of the raw powder was reduced by boiling in acetone. The surfactant was largely removed. The total carbon content was still

2.7 + 0.1%. In a first compression step, the powder was pressed with the surfactant as a binder in a steel matrix under 20 kN. (130 MPa) to a green body of 40 - 45% of the theoretical density. In the subsequent first tempering step up to 400 ° C the total carbon content was reduced to 1.23 + 0.04%. The pressing was then cold isostatically compressed to a density of 55-60% at 550 MPa. The presence of residues of organic carbon proved to be beneficial, since premature agglomeration of the particles was suppressed. The remaining binder was burned out in air at 650 ° C. The residual total carbon content, which was essentially carbonate, was 0.29 + 0.02%. In order to obtain a maximum density of the finished sintered body, it was welded into a noble metal capsule. The capsule with the pressing was first cold isostatically pressed at 400 MPa in order to ensure that the metal was pressed tightly against the pressing. Finally, the sample was sintered at 850 ° C and 200 MPa for 15 minutes. It became a ceramic with a density of 98 ± 0.6% of the theoretical density and an average grain size of 50.4 nm were obtained.

Particularly advantageous ceramics which can be prepared with the inventions to the invention method, the ceramics Ba ₃ and Ti0 KNaNb ₂ 0 ₆ described in the embodiments, but also BaFe ₁₂ 0ι ₉

Both oxides belong to the class of ferroelectrics. They have a structural phase transition from a ferro-electrical low-temperature phase to a paraelectric high-temperature phase, which is generally associated with a very strong dependence of the dielectric constants of the material on the temperature. Around the range of the Curie temperature T (BaTi0 ₃ : T _c = 120 ° C, KNaNb0 ₆ : T _c = 425 ° C), which characterizes the transformation point, both the dielectric and the piezoelectric coefficients have maximum values that both drop drastically for lower and higher temperatures.

Literature cited:

[1] M. Ciftcioglu, M. J. Mayo; "Processing of

Nanocrystalline Ceramics "; Mat. Res. Soc. Symp. Proc. Vol. 196 (1990) 77-86

[2] W. Härtl. Ch. Beck, M. Roth, F. Meyer, R.

Hempel man; "Nanocrystalline Metals and Oxides II: Reverse Microemulsion"; Ber. Bunsenge. Phys. Chem. 101, 1714 -1717 (1997) [3] ff. Herrig, -R. Hempel man; "A colloidal approach to nanometer-sized mixed oxide ceramic powders"; Materials Letters, vol. 27, 287 (1997) [4] Ch. Beck, W. Härtl, r. Hempel man; "Size-controlled synthesis of nanocrystalline BaTiö ₃ by a sol-gel type hydrolysis in microemulsion-provided nanoreac tors"; J. Mater. Res. , Vol. 13, No. 11, (1998) 3174-3188.

[5] Amini, sacks; J. American Ceramic Society 74 (1), 53-59 (1991)

[6] R. Waser et al. : J. Am. Ceram. Soc. 73 (1990) 1645; R. Waser: Ferroelectrics 133 (1992) 109)

Claims

claims

1. A method for producing a ceramic, which has a density of at least 94% of the theoretical density, with the steps - the starting powder for the ceramic is produced with the aid of a microemulsion, - the starting powder thus produced is solidified by a pressure-assisted process to the ceramic.

2. The method according to the preceding claim 1, in which a starting powder is produced in which less than 10% of the particles have a size of more than three times the average size.

3. The method according to the preceding claims 1 to 2, in which a starting powder is produced in which less than 5% of the particles have a size of more than three times the average size.

4. The method according to the preceding claims 1 to 3, in which a starting powder is produced in which less than 2% of the particles have a size of more than three times the average size.

5. The method according to the preceding claims 1 to 4, in which a starting powder is produced which has an average particle size of less than 100 nm.

6. The method according to the preceding claims 1 to 5, in which a starting powder is produced which has an average particle size of less than 50 nm.

7. The method according to any one of the preceding claims 1 to 6, in which a starting powder is produced which has an average particle size of less than 10 nm.

8. The method according to any one of the preceding claims 1 to 7, in which BaTi0 _{3 is produced} as the starting powder.

9. The method according to any one of the preceding claims 1 to 7, in which a starting powder comprising niobium is produced.

10. The method according to any one of the preceding claims 1 to 7, in which KNaNb ₂ 0 _{6 is produced} as the starting powder.

11. The method according to any one of the preceding claims 1 to 10, in which the starting powder is produced which has amorphous particles.

12. The method according to any one of the preceding claims 1 to 12, in which a microemulsion with a polar oil is used.

13. The method according to any one of the preceding claims 1 to 12, in which the starting material is solidified by gas pressure sintering.

14. The method according to any one of the preceding claims 1 to 12, in which the starting powder is solidified by means of hot pressing.

15. The method according to any one of the preceding claims 1 to 12, in which the starting powder is solidified by means of sinter forging.

16. High-density ceramic, can be produced by a method according to claims 1 to 15, characterized by

- a density which corresponds to at least 94% of the theoretical density, and

- With an average grain size of less than 500 nm.

17. High-density ceramic according to claim 16 with an average grain size of less than 100 nm.

18. High-density ceramic according to claim 16 to 17 with an average grain size of less than 50 nm.

19. High-density ceramic according to claim 16 to 18 with a narrow grain size distribution, the proportion of grains with more than three times the average grain size is preferably less than 20%.

20. High-density ceramic according to claim 16 to 18 with a narrow grain size distribution, the proportion of grains with more than three times the average grain size is preferably less than 10%.

21. High-density ceramic according to claim 16 to 20 with a density which corresponds to at least 97% of the theoretical density.

22. High-density ceramic according to claim 16 to 21 with a density which corresponds to at least 99% of the theoretical density.

23. High-density ceramic according to claim 16 to 22, comprising BaTi0 ₃ .

24. High-density ceramic according to claim 16 to 22, comprising niobium-containing oxides.

25. High-density ceramic according to claim 24, comprising KNaNb ₂ 0 ₆ .