Patent Application
20250197291
The present invention concerns the manufacture of ceramic composite parts from a mixture of mineral powders of different sizes and/or densities.
Examples include parts made of alumina-rutile composite material, where the alumina powder has a median particle size (D50) of 0.2 to 0.4 micron and the rutile powder has a median particle size (D50) of 1 to 2 microns.
Such ceramic composite parts can be obtained by synthesizing the powder from a solution with a mixture of precursors. Such a process is very costly.
Such ceramic composite parts can also be obtained by preparing an aqueous suspension of a mixture of starting mineral powders, incorporating at least one binder and/or at least one plasticiser into the aqueous suspension, atomising the resulting aqueous suspension in order to obtain granules, shaping the granules by pressing, and heat-treating the resulting preform, called sintering, in order to consolidate the desired part, the binder(s) and/or and densify plasticiser(s) being pyrolyzed by the sintering.
With this process, which requires the production of large batches of powders due to the atomisation process, we observe significant drift in the chemical composition of composite parts, as well as risks of discharge or pollution due to the fine powder particles produced. This significant drift in composition is linked in particular to granulometric segregation phenomena during the atomisation operation, due to the different sizes of the starting powders.
Current processes either are very expensive to guarantee precise, controlled chemical composition of composite parts, without pollution, or are unsuitable for precise control of the chemical composition of composite parts.
Until now, industrial needs have been met by integrating chemical composition drifts during the process into the starting composition.
This approach requires a posteriori knowledge of process drift, which is very difficult to determine with any degree of precision.
La Lumia et al. “Fabrication of homogenous pellets by freeze granulation of optimized TiO2—Y2O3 suspensions” Journal of European Ceramic Society vol. 39, No 6, 2019 and patent application WO 2019/038497 A1 describe a process for manufacturing homogeneous pellets for nuclear applications. Marco D et al. Dielectric properties of alumina doped with TiO2 from 13 to 73 Ghz” Journal of European Ceramic Society, vol 37, No 2, 2016, Miyauchi et al. “Improvement of the dielectric properties of rutile doped Al2O3 ceramics by annealing society” Journal of European Ceramic Society, vol. 26, No 10-11, 2006 and HUANG CHENG-LIANG ET AL: “Microwave Dielectric Properties of Sintered Alumina Using Nano-Scaled Powders of alpha-Alumina and TiO2”, Journal of the American Ceramic Society, vol 90, no 5, May 2007, pp 1487-1493 describe conventional processes for preparing mixtures of alumina and titanium oxide that can lead to heterogeneities in the powder mixture and, above all, to pollution.
Parts made of ceramic composite material according to the present invention are intended in particular to constitute:
The areas identified include microwave telecommunications for communications satellites, civil and military ground and airborne radars, microwave components for civil telecommunications such as 4G, 5G and post-5G networks, and PMR (Professional Mobile Radio) networks on free and licensed frequency bands.
For such ceramic parts, low porosity, low pollution and good phase homogeneity are necessary to obtain the desired properties.
The aim of the present invention is therefore to make it possible to manufacture composite ceramic parts from ceramic base powders that are highly sensitive to changes in their properties due to possible chemical or compositional contamination of the material during production.
For this purpose, it is proposed, according to the present invention, to treat an aqueous suspension of a mixture of starting powders with the binder(s) and/or plasticiser(s) by cryogenic granulation, making the process easy to implement and enabling control of the chemical composition of the material during its production by limiting the risks of pollution or drift of the chemical composition.
Unlike atomisation, cryogenic granulation is easy to implement, fast and reproducible, avoiding the need to manufacture large batches of powder, which is a source of the drifts described above, and leading to the desired properties: high density, closed porosity, dielectric properties.
More specifically, we were able to obtain good homogeneity of the starting powders within the granules, limiting the migration of smaller particles, a wide distribution of granule sizes, good compactability and excellent natural sinterability.
The first object of the present invention is therefore a method for producing a ceramic part from a mixture of two pulverulent mineral materials which differ in terms of their size and which consist of alumina and titanium oxide, characterized in that:
The starting pulverulent mineral materials can be chosen from those with median particle sizes (D50) of between 0.1 and 10 microns and densities of between 1 and 7 g/ml.
By way of example, the starting alumina may have a median particle size (D50) of 0.2 to 0.4 μm and the starting titanium oxide introduced in particular in its rutile form, may have a median particle size (D50) of 1 to 1.6 μm.
In step (a1), the mixture of pulverulent mineral materials may be present at a content of 5 to 30% by volume relative to the volume of water in the suspension.
In step (a1), at least one dispersant can be used which represents, on a dry basis, 0.2 to 0.5% by weight of the mixture of the starting pulverulent mineral materials.
In step (a1), a 1% by weight aqueous solution of poly(ammonium methacrylate) can be used as the dispersant.
In step (a2), the binder can be chosen from the main binders used in aqueous media, such as polyvinyl alcohol (PVA), latex emulsions and cellulose binders such as carboxymethylcellulose (CMC).
In step (a2), the plasticiser can be chosen from glycols, being for example polyethylene glycol.
The quantity by weight of binder(s) and/or plasticiser(s) may represent 1 to 5% by weight of the powder in the suspension.
In step (a1), the mixing can be carried out using a 3D dynamic mixer using beads with a diameter of 2 mm and three times the volume fraction of the powder, with mixing taking a few hours.
In step (b), the suspension obtained in step (a2) is advantageously pumped to an injection nozzle in a cryogenic enclosure in order to ensure pulverization of the suspension in said cryogenic enclosure, in order to obtain compact spherical granules with a size distribution advantageously between 10 μm and 1 mm.
In step (c), the granules can be uniaxially or isostatically pressed at a pressure of between 30 and 200 MPa.
In step (d), the sintering can be carried out at a temperature of between 1300° C. and 1500° C.
The process according to the present invention can lead to a ceramic part in the form in particular of a block, machinable block, strip, cuttable strip having at least one of the following properties:
The relative permittivity (εr) is determined by extracting the complex permittivity of a dielectric resonator in a cylindrical cavity.
The dielectric loss expressed by the loss angle defined by tan δ is as determined by extracting the complex permittivity of a dielectric resonator in a cylindrical cavity.
The linear temperature stability coefficient (τf) is determined by measuring the resonant frequency of a dielectric resonator in a cylindrical cavity as a function of temperature in a climatic chamber.
Another object of the present invention is the use of the ceramic part obtained by the method as defined above as a constituent of all or part of microwave components or systems using ceramics in the telecommunications sector, these components being able to:
The following Examples illustrate the present invention without, however, limiting its scope.
Preparation of a Composite of Alumina (Al2O3) Doped with Titanium Dioxide (TiO2) in the Rutile Phase for the Manufacture of a High-Frequency Filter
0.2 g of an aqueous solution of poly(ammonium methacrylate), marketed by Vanderbilt Minerals, LLC as DARVAN® C-N, was placed in a plastic container containing approx. 119 g of 2 mm-diameter alumina beads.
The container was then filled successively with:
The alumina powder is an alumina powder marketed by Taimei with a median particle size (D50) of 0.2 μm.
The rutile powder is a rutile powder with a median particle size (D50) of 1.6 μm.
The resulting suspension was mixed for 3 hours using a Turbula® Type T2C three-dimensional agitator (Willy A. Bachofen A.G., Switzerland), which deagglomerated the alumina and rutile powders and ensured their homogeneous mixing.
(a2) Homogeneous Mixing of the Suspension from Step (a1) with a Binder and a Plasticiser
Once mixing was complete, the alumina beads were removed from the container and a mixture of:
The resulting suspension was rotated on a roller agitator for at least 3 hours at a speed of 35 rpm, homogenizing the suspension and integrating the plasticiser until a fluid suspension was obtained.
Stage (b): Cryogenic Granulation of the Fluid Suspension from Stage (a2)
This device enables a freezing by spraying to be quickly followed by lyophilisation of the frozen granules, during which the granules are dried by sublimation of the ice they contain, without altering their sphericity. The device comprises:
The suspension was injected at a peristaltic pump-controlled speed of 33 ml/min and a specific air pressure of 0.2 bar.
A lyophilisation operation took at least 6 hours for the volume of suspension obtained in this example, during which a stable vacuum of 10−3 mbar and a temperature of around −100° C. were ensured.
The granules obtained during cryogenic granulation have good sphericity and a homogeneous distribution of alumina and titanium particles.
The powder, compressed into pellets, was subjected to a sintering heat treatment at 1400° C. The sintered pellets obtained show good phase homogeneity (see
This figure shows the presence of the phases corresponding to alumina and rutile. In addition, no secondary phases are observed.
This micrograph shows a dense composite, with a very low porosity rate (in black) of less than 2%. This porosity rate was measured by image analysis on micrographs of the composite. The micrograph also shows the presence of alumina (in dark gray) and rutile grains (in light gray) perfectly dispersed in the alumina matrix. Rutile distribution can be seen mainly in the alumina grain boundaries.
This example illustrates the advantages of using the cryogenic granulation process of the invention over the conventional spray granulation process. The composite manufacturing process of Example 1 has been compared with the spray granulation process used for a mixture of Al2O3 and TiO2 powders.
Three compositions with 0.5, 10 and 12% TiO2 were produced by the granulation method of Example 1 and by atomisation.
For each composition, 150 g of the mixture is introduced into a plastic jar with 300 g of 0.8 mm YTZ beads, the same dispersant as used in Example 1 and osmosis water. These components are mixed in an attritor mill at 800 rpm for 30 min.
Once mixing is complete, the YTZ beads are removed from the jar and the same binder-plasticiser mixture is added to the suspension in the same ratio as used in Example 1.
The resulting suspension is rotated by means of a roller agitator at a speed of 35 rpm, to homogenize the suspension and integrate the binder and plasticiser. Agitation is maintained for at least 3 hours until a fluid suspension is obtained.
Each resulting suspension was atomised in a Büchi Mini Spray Dryer B-290. Atomisation conditions were set at 80% suction, 170° C. inlet temperature, 10% pumping and an air flow rate of around 40 ml/min.
This powder then underwent pressing and sintering steps similar to those described in Example 1.
Table 1 shows the compositions achieved by each granulation method, as well as the amount of TiO2 measured before and after granulation.
A reduction in the percentage of TiO2 of around 3% was observed in composites that had been granulated by atmoisation. In addition, the dielectric properties of samples granulated by cryogenic granulation outperform those of samples granulated by atomisation (tan δ of 10−5 and tan δ of 10−4 respectively).
| Number | Date | Country | Kind |
|---|---|---|---|
| 2111783 | Nov 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2022/060543 | 11/2/2022 | WO |
