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Definitions

• the present invention relates to porous ceramic bodies and a process for preparing said porous bodies. It also relates to their uses, in particular for the preparation of composite materials.
• porous bodies are described containing particles of Si ⁇ 2, AI2O3, ZnO, Zr ⁇ 2, MgO, PbO, B2O3, SÎ3N4, BN or AIN whose average size does not exceed 10 ⁇ m. These porous bodies are obtained by shaping a ceramic powder and sintering. After infiltration by a resin, the porous bodies lead to composite materials which can be used in the field of electronics.
• porous ceramic bodies preforms reinforced with substantially spherical AIN particles of size between 10 and 100 ⁇ m. These porous bodies can be used to manufacture composite materials with a metal matrix.
• porous ceramic bodies having now been found, said porous bodies being characterized in that they comprise particles of aluminum nitride (AIN) having an aspect ratio at least equal to 5.
• AIN aluminum nitride
• the term ratio of aspect is here used in its conventional sense, namely that it designates the diameter / thickness ratio.
• the invention relates more particularly to porous bodies whose porosity is at least equal to 60% by volume.
• the invention particularly relates to porous bodies made up of at least 60% by weight of AIN.
• the invention also relates to mixed porous bodies in which said particles are associated with one or more other reinforcing products such as whiskers, short fibers, fine ceramic particles, the AIN content preferably remaining in the majority.
• the invention also relates to a process for preparing porous ceramic bodies. This process by carbonitriding from alumina is characterized in that alumina particles are used having an aspect ratio at least equal to 5. According to a first preferred variant, the process is carried out using an alumina powder and carbon.
• the alumina powder is generally chosen from powders whose particles are in the form of tabular crystals or fibers.
• ⁇ alumina crystals are used which mainly have the appearance of polygonal plates, and advantageously hexagonal, and which have a size varying from 2 to 50 ⁇ m and preferably less than 15 ⁇ m and a thickness varying from 0.1 at 3 ⁇ m and preferably less than 1.5 ⁇ m.
• Such crystals can be obtained for example by calcination of an ⁇ alumina precursor in the presence of flux according to the method of preparation described in application EP 0425325 in the name of the Applicant.
• Carbon is generally chosen from lamp black, smoke black, tunnel black, oven black, activated carbon, carbon or graphite felt and graphite powder.
• carbon precursors such as hydrocarbons, the thermal decomposition of which leads to carbon deposition.
• saturated linear hydrocarbons such as methane or unsaturated hydrocarbons such as ethylene and acetylene or aromatics.
• the process of the invention is generally implemented, by mixing amounts of carbon and the alumina such that the carbon / alumina molar ratio is between 2 and 20 and preferably 2.8 and 10. Values of the molar ratio greater than 20 are not of interest because they lead to macrocrystals containing a large excess of residual carbon, the elimination of which proves to be expensive.
• the mixture comprising alumina and carbon generally undergoes a shaping step which can be, for example, an extrusion, an injection molding or an isostatic or uniaxial pressing.
• the method of the invention is implemented using a porous body containing said particles of alumina and carbon.
• a porous body based on alumina has a porosity at least equal to 55% by volume and preferably 70%.
• the porous body is covered with carbon consisting, for example, of carbon black or graphite.
• the porous body is subjected to carbon infiltration.
• such an infiltration is carried out using the resin generating carbon by pyrolysis described above in liquid, molten, or dissolved or emulsified form.
• the process is generally carried out in the presence of nitrogen and / or a nitrogen-generating gas such as ammonia.
• the carbonitriding reaction is generally carried out at a temperature between 1350 and 1900 ° C, and preferably between 1400 and 1600 ° C, and for a time sufficient to obtain a porous body based on AlN. For information purposes only, this time can vary from 30 minutes to 15 hours.
• the residual carbon can optionally be eliminated by combustion in air at a temperature between 500 and 800 ° C.
• the porous body which is the subject of the invention is capable of numerous applications. Mention may in particular be made of its use for preparing composite materials, said materials being obtained by infiltration of the porous body, for example by a thermosetting polymer in the liquid state or in solution or thermoplastic in the molten state or in solution, a molten metal or a ceramic precursor in colloidal solution or in vapor phase.
• the porous body according to the invention can, thanks to its low wettability by metals and its high resistance to corrosion by molten salts, constitute an excellent filter for molten metals.
• acetylene black (Y50, SN2A), 74.8 g of formophenolic resin (R3593, CECA) and 90.0 g of alumina in the form of platelets are introduced. (grade Tj, Elf Atochem) in a Z-arm mixer.
• the alumina wafers consist of monocrystals of alumina ⁇ in the form of more or less regular polygons (mainly hexagons) having an average diameter between 5 and 10 ⁇ m, a thickness between 0.2 and 0.6 ⁇ m and an aspect ratio of approximately 20.
• the mixture is extruded to form cylindrical granules having a diameter of 3 mm which are then air dried at 150 ° C in order to polymerize the resin.
• Porous bodies are obtained with a geometry similar to that of the starting granules and the porosity of which is equal to 70% (porosity calculated from the determination of the apparent density of the preform knowing the absolute density of the material). Analysis of these porous bodies using a scanning electron microscope shows that the AIN obtained consists of irregularly-shaped platelets, sometimes pierced, with morphological characteristics and dimensions close to those of the starting alumina ( Figure 1). The oxygen content of the porous body determined by X-ray fluorescence is equal to 1.1%, which corresponds to an almost total transformation of the alumina into AIN.
• EXAMPLE 2 5.2 g of acetylene black (Y50, SN2A), 6.3 g of formophenolic resin (R3593, CECA) and 18.0 g of alumina in the form of platelets (T'o grade, Elf) are introduced Atochem) in a knife mixer. These wafers are ⁇ alumina single crystals, of polygonal shape with a hexagonal majority having a diameter between 2 and 7 ⁇ m and a thickness between 0.1 and 0.5 ⁇ m. The mixture is pressed at 30 bars to form a pellet which is dried at
• Example 1 150 ° C in a ventilated oven.
• the pellet is introduced into the oven of Example 1 heated to 1550 ° C under nitrogen (34 l / h) for 10 h. After natural cooling of the oven, the excess carbon from the pellet is eliminated by combustion in air at 700 ° C.
• a porous body (pellet) is obtained, the percentage of AIN, evaluated according to the weight losses, is close to 100%.
• the porosity of the porous body is equal to 71%.
• the AIN obtained is in the form of plates of diameter between 2 to 7 ⁇ m and having an aspect ratio equal to 6. The plates have a very irregular surface and some are pierced.
• Example 2 The procedure is carried out under the conditions of Example 2 in the presence of alumina in the form of T2 grade platelets (Elf Atochem).
• alumina in the form of T2 grade platelets (Elf Atochem).
• These polygonal ⁇ alumina plates mainly hexagonal, have a diameter between 10 and 16 ⁇ m, a thickness between 0.7 and 1.2 ⁇ m and an aspect ratio between 10 and 20.
• a porous body is obtained (pellet) having a porosity equal to 70.7% consisting of AIN in the form of platelets of geometry similar to that of the starting alumina.
• the conversion rate of alumina into AIN, evaluated by weighing, is 95%.
• the surface of the AIN platelets is irregular and we note that some platelets have perforations.
• a porous body consisting of alumina in the form of T'Q grade platelets (Elf Atochem) obtained according to the method of preparation described in European patent application EP 0 460 987 is used.
• the porous body (6.15 g; porosity: 78%) is introduced into a container which can be evacuated equipped at its upper part with a dropping funnel filled with formophenolic resin (R 3593, CECA). After creating a vacuum in the container, the porous body is infiltrated with the resin. The impregnated porous body is dried at 150 ° C and subjected to pyrolysis at 900 ° C under a nitrogen atmosphere. The variation in weight of the porous body indicates that 2 g of carbon have been deposited.
• the porous impregnated body is placed in the tubular furnace of Example 1 at a temperature of 1550 ° C. and under a nitrogen flow rate of 30 l / h for 12 h. Excess unreacted carbon is removed by combustion in air at
• the weight fraction of AIN in the product is equal to 70%.
• a porous body is obtained having a porosity of 76% and consisting of AIN in the form of polygonal plates of irregular surface having a diameter between 2 and 7 ⁇ m, a thickness between 0.1 and 0.5 ⁇ m and a ratio d 'aspect equal to 6.
• the weight fraction of AIN in the product is 63%.
• a porous body of porosity equal to 80% is obtained, consisting of AIN in the form of polygonal plates of irregular surface having a diameter of between 12 and 16 ⁇ m, a thickness of between 0.7 and 1.2 ⁇ m and a ratio of aspect between 10 and 20.
• Example 4 The procedure is carried out under the conditions of Example 4 in the presence of a porous body weighing 8.2 g (porosity: 79.5%) and a furan resin (LQ 1300, Quaker Oats Chemicals). As the viscosity of the resin is high, the porous body is impregnated with the resin in a device maintained at a temperature of 70 ° C. The weight fraction of AIN in the product is 61%.
• a porous body with a porosity equal to 79% is obtained, consisting of AIN in the form of polygonal plates with curvilinear edges similar to those described in Example 4.
• a porous body consisting of alumina is used in the form of T'O grade platelets (Elf Atochem) prepared according to the embodiment described in patent application EP 0 460 987.
• the porous body (6.7 g) is introduced into the tube furnace of Example 1 heated at 1550 ° C for 12 h under the current of a gas mixture consisting of nitrogen and methane (90:10 v / v) .
• the excess carbon resulting from the decomposition of methane is eliminated by combustion in air at 650 ° C.
• the change in weight during the heat treatment indicates an AIN content of 61%.
• a porous body is obtained having a porosity equal to 70% consisting of AIN having characteristics similar to those described in Example 4.
• porous body consisting of alumina in the form of T2 grade platelets (Elf Atochem) obtained according to the method of preparation described in European patent application EP 0460987.
• the porous body (2.9 g; porosity: 80%) is placed between two layers of graphite felt (RVG 4000, Carbone Lorraine) and the assembly is introduced into a sintering oven, the atmosphere of which can be controlled.
• the air from the sintering furnace is expelled and replaced with nitrogen.
• the oven is brought to 1800 ° C. in 2 hours and maintained at this temperature for 4 hours. After cooling, it is found that part of the carbon felt surrounding the preform has disappeared.
• the porous body obtained analyzed by X-ray diffraction, consists of 99% AIN.
• the powder obtained is in the form of polygonal plates of irregular surface having a diameter between 10 and 16 ⁇ m, a thickness between 0.7 and 1.2 ⁇ m and an aspect ratio between 10 and 20 .
• the procedure is carried out under example 4 in the presence of a porous Saffil® body (HERE) made up of amorphous alumina fibers having a diameter between 1 and 4 ⁇ m.
• the porous body (3.5 g; porosity: 84%) is impregnated with a furan resin (LQ 1300, Quaker Oats Chemicals) under the conditions described in Example 6. 2 g of carbon are thus deposited on the porous body.
• porous body with a porosity equal to 82% is obtained, consisting of AIN at 64%. Examination by scanning electron microscopy of a section of this porous body shows that it is made up of fibrils comprising an outer sheath of irregular appearance and a core ( Figure 2).
• the porous body After coating with a resin and polishing, the porous body is subjected to an examination by scanning electron microscopy and probe with light element probe (Figure 3).
• the elements Al and O appear in yellow and the elements Al and
• the fibers have a sheath rich in nitrogen and a core rich in oxygen.

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