Classifications

• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
  • C04B35/645—Pressure sintering
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/90—Carbides
  • C01B32/914—Carbides of single elements
  • C01B32/991—Boron carbide
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B28—WORKING CEMENT, CLAY, OR STONE
  • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
  • B28B13/00—Feeding the unshaped material to moulds or apparatus for producing shaped articles; Discharging shaped articles from such moulds or apparatus
  • B28B13/02—Feeding the unshaped material to moulds or apparatus for producing shaped articles
  • B28B13/0205—Feeding the unshaped material to moulds or apparatus for producing shaped articles supplied to the moulding device in form of a coherent mass of material, e.g. a lump or an already partially preshaped tablet, pastil or the like
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B28—WORKING CEMENT, CLAY, OR STONE
  • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
  • B28B3/00—Producing shaped articles from the material by using presses; Presses specially adapted therefor
  • B28B3/02—Producing shaped articles from the material by using presses; Presses specially adapted therefor wherein a ram exerts pressure on the material in a moulding space; Ram heads of special form
  • B28B3/025—Hot pressing, e.g. of ceramic materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B28—WORKING CEMENT, CLAY, OR STONE
  • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
  • B28B3/00—Producing shaped articles from the material by using presses; Presses specially adapted therefor
  • B28B3/02—Producing shaped articles from the material by using presses; Presses specially adapted therefor wherein a ram exerts pressure on the material in a moulding space; Ram heads of special form
  • B28B3/04—Producing shaped articles from the material by using presses; Presses specially adapted therefor wherein a ram exerts pressure on the material in a moulding space; Ram heads of special form with one ram per mould
  • B28B3/06—Producing shaped articles from the material by using presses; Presses specially adapted therefor wherein a ram exerts pressure on the material in a moulding space; Ram heads of special form with one ram per mould with two or more ram and mould sets
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
  • C04B35/563—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on boron carbide
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/60—Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
  • C04B2235/604—Pressing at temperatures other than sintering temperatures
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
  • C04B2235/656—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes characterised by specific heating conditions during heat treatment
  • C04B2235/6562—Heating rate
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
  • C04B2235/656—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes characterised by specific heating conditions during heat treatment
  • C04B2235/6567—Treatment time
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
  • C04B2235/66—Specific sintering techniques, e.g. centrifugal sintering
  • C04B2235/666—Applying a current during sintering, e.g. plasma sintering [SPS], electrical resistance heating or pulse electric current sintering [PECS]
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
  • C04B2235/74—Physical characteristics
  • C04B2235/77—Density
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
  • C04B2235/96—Properties of ceramic products, e.g. mechanical properties such as strength, toughness, wear resistance
  • C04B2235/9607—Thermal properties, e.g. thermal expansion coefficient
  • C04B2235/9623—Ceramic setters properties

Definitions

• the field of the invention is that of its manufacture of high density sintered material elements, and in particular ceramic elements boron carbide (B 4 C).
• Materials made by sintering powders may contain a residual porosity rate. The actual density of the sintered material is then less than its theoretical density.
• the theoretical density corresponds to the density of the single crystal and can be calculated from the chemical composition and the crystalline structure.
• Relative density or actual density, expressed as a percentage of the theoretical density, includes porosity, defects in the crystal lattice, and secondary phases.
• B 4 C boron carbide
• B 4 C pellets for nuclear use must have a relative density of at least 96 % and a substantially closed porosity, to avoid possible entrapment of sodium.
• the HP method achieves the minimum required relative density of 96%, but with little margin, which can lead to a significant and costly non-compliance rate (especially if the B 4 C is enriched with 10 ⁇ ) and the presence of open porosity.
• the microstructure of the sintered materials obtained by HP sintering can change, and in particular the grain size, which can lead to a reduction in the mechanical performance of the sintered material in comparison with materials with a finer microstructure .
• SPS Spark Plasma Sintering
• the main difference between the HP sintering method and the SPS sintering method is in the manner of heating the powder compact.
• the heating is indirect: it is done through the internal resistances of the oven. The sample is heated by thermal conduction from the pressing die to the powder compact.
• SPS sintering the heating is direct: a pulsed electric current passes through the compression tool (die and pistons) and / or the powder compact to be sintered, thus ensuring heating by Joule effect and conduction.
• SPS sintering makes it possible to obtain denser materials and finer microstructures than with HP sintering. This last point is due to the reduction of sintering cycle times and the reduction of the temperature plateau time in SPS sintering compared to HP sintering.
• SPS sintering thus offers the possibility of optimizing the microstructures of the sintered material, while significantly reducing the cycle times compared to HP sintering.
• SPS sintering is performed primarily by means of a matrix mono-imprint, which allows the unit manufacture of pellet or slab, and is therefore not suitable for manufacturing on an industrial scale.
• the inventors have set themselves the objective of achieving in a simple, fast and efficient way sintered materials having a finer microstructure and a higher density than those produced by the HP method, leading to a decrease in the presence of an open porosity, even its removal, and an improvement in the mechanical properties of the sintered material.
• This last point is particularly important in the case of B 4 C pellets intended for nuclear use, since an improvement in the mechanical properties of the material would make it possible to offer better resistance to the thermal gradient in the reactor, a bad resistance constituting the main cause of mechanical degradation of the sintered material in operation,
• the inventors have retained the SPS method, making improvements. Indeed, as we have just seen, the SPS method allows a unit piece manufacturing (single-impression tooling), which is incompatible with industrial mass production. Unlike HP sintering, where the use of multi-cavity dies is common and easy because the heating is performed by a peripheral heating element, in SPS sintering, this type of tooling exists but requires, in the case of our application to high temperatures, adaptations of the process. The first tests carried out in the laboratory, without these adaptations and with conventional HP-type multi-cavity matrices, have also shown the presence of density and microstructure heterogeneities in the pellets of sintered material, with local formation of craters due at the passage of the current.
• the invention therefore relates to a process for manufacturing pellets of sintered material, comprising the following successive steps:
• a compression tool (which may also be called a compaction or pressing tool) comprising a die having a plurality of cavities and. pistons for sliding in the cavities;
• the first and the second compaction are obtained by the application of a uniaxia force.
• the maximum density of raw material reached by the pulverulent material is the maximum density that a powder can reach under the effect of a pressure applied to it.
• its first and second sintered boron nitride disks have a purity greater than or equal to 99.7%,
• the cavities of the matrix are distributed symmetrically and equidistant from the periphery.
• the cavities may for example be arranged on a circle 13 centered on the axis of the cylinder, at equal distances from each other, as illustrated in Figure 1b, where the matrix has four cavities. This configuration allows a better temperature homogeneity.
• the method furthermore comprises, between step b) and step c), a step of covering, by a graphite film, an inner wall of each cavity. intended to be in contact with the precompacts, and a contact surface of the pistons intended to be in contact with one of the first and second boron nitride disks.
• the graphite film prevents pollution of the cavities of the matrix and facilitates the demolding of the sintered compacts. It may for example be a tiebook or a graphite disc sold under the trade name Papyex TM by the MERSEN company.
• the method further comprises, between step b) and step c), another step of covering, by a graphite film, the interfaces between the precompacts and the first boron nitride disks, and interfaces between the pre-compacts and the second boron nitride disks.
• the graphite film may be a Papyex TM graphite disc; it makes it possible to limit any adhesion of the boron nitride of the first and second discs to the surface of the sintered pellets.
• the sintered material is boron carbide (84C).
• FIG. 1a is a diagrammatic representation of an example of a piston and a compression matrix comprising four cavities in a perspective view;
• Figure 1b is a schematic representation of the compression matrix of Figure la, according to a view from above;
• FIG. 2 is a sectional view of a compression tooling ready to be inserted into an enclosure of an SPS device;
• FIG. 3 represents a partial sectional view of an example of an SPS sintering device for implementing the method that is the subject of the invention.
• the process according to the invention makes it possible to produce pellets of high density sintered material by the SPS sintering method in a multi-cavity matrix. This method is particularly useful for the industrial scale manufacture of high density 84C pellets for nuclear use. These high density B 4 C pellets can in particular be used as neutron absorbents, particularly in fast neutron 4 th generation reactors.
• precompacts are calibrated from powder portions (of equal masses, weighed by means of a high precision balance) by a first uniaxial pressing at a first threshold below the maximum density threshold. in the raw state reached by the powdery material, and then introduced into the multi-cavity matrix.
• a second uniaxial pressing of the pre-compacts is then performed at a second threshold which is greater than the first threshold and which is close to its maximum density reached in the raw state by the powder material in a multi-cavity matrix.
• the second threshold is located closest (if possible equal) to the maximum density threshold reached in raw by the powder material.
• Each of the compact calibrated and has an equivalent level of density.
• the first pressing is preferably carried out in a single-cavity matrix, but it could very well be done in the matrix mufti-imprints used for the second pressing.
• the compression tooling is balanced and the height of the pistons is constant.
• This allows the application of an equivalent pressure (homogeneous) on all the pre-compacts during the advance of the pistons during the SPS heat treatment sinter, It also ensures a distribution of the most homogeneous current possible and therefore a more uniform temperature distribution, especially for voluminous compressive oufflages.
• this second compaction ensures the maintenance of the multi-cavity matrix on the lower pistons, without it slipping when it is in the vertical position.
• the compacts indeed support the inner faces of the cavities of the multi-cavity matrix.
• boron nitride (BN) sintered discs having a thickness millimeter (for example, 2.5 mm thick) and high purity ( ⁇ 99.7%) are used.
• BN boron nitride
• the presence of these discs guarantees the absence of current flow from the pistons to the pre-compacts (no hot spot), including for very strong currents because of the significant thickness of the BN discs compared to that of the disc. a deposit that would be made by spraying a suspension.
• the current will indeed bypass the boron nitride disks, the disks forming an electrically insulating layer, including at very high temperature.
• the distribution of the current being controlled and homogeneous, the pre-compact temperature is more homogeneous, it results in a better homogeneity in terms of density and microstructure (grain size) throughout the volume of the pellets.
• the dense nature of the discs ensures their mechanical strength and greater robustness in comparison with the cohesive deposits made by spraying.
• the millimetric thickness of the discs allows the discs to withstand SPS sintering temperatures, that is, temperatures above 1000 ° C, which can cool down to 2000 ° C under a controlled atmosphere.
• a dense disc and millimeter thickness avoids a possible pollution of the compact, because it is much less fragile.
• a single-cavity stainless steel matrix provided with a through-mold cavity, and two stainless steel pistons, intended to slide in the cavity of molding to compact the powder in the cavity and thus form the precompacts 7.
• a compression tool in graphite comprising a matrix muiti-imprints 1 provided with four molding cavities 2 traversing distributed so symmetrical and circular within the matrix, as illustrated in Figure 1b, and eight pistons 3 ⁇ a single piston being shown in Figure la) for sliding in the cavities 2 to compact the pre-compacts 7 in the cavities and thus forming calibrated compact shapes of desired shape and size.
• the pistons operate in pairs and move in opposite directions to each other during compression.
• the cavity of the single-cavity matrix has a diameter of 20 mm and the cavities of the multi-cavity matrix 1 have a diameter of 20.4 mm, the difference making it possible to take into account the thickness of 200 ⁇ of Papyex TM.
• the four cavities of the multi-cavity matrix have a height of 120 mm.
• a B, 3 C powder from H, C. Starck HS type was used as a raw material and weighed to form four portions of the same mass (in this case 15.60 g).
• one of the powder portions was introduced into the cavity of the single-cavity stainless steel compression matrix and a first uniaxial compaction at low pressure ( ⁇ 10 MPa) was carried out at a lower density. at the maximum value achievable by the powder, in this case 1 MPa for 1 minute. These operations were repeated for each of the powder portions,
• the pre-compacts 7 thus obtained are then extracted from the single-cavity matrix and stored until they are used subsequently.
• a Papy ex TM 4 sheet was cut to the internal dimensions of the inner side wall of each of the cavities of the multi-cavity die 1 and was placed on this inner side wall in each cavity.
• the lower four pistons 3 are inserted into the lower part of the cavities and a Papyex TM disk 6 of the same size as the lower piston head is placed in the bottom of each cavity.
• the four pre-compacts 7 are introduced into the cavities 2 of the matrix.
• a disc of Papyex TM 6 is placed at the bottom and at the top of the pre-compacts 7.
• An upper disk of type AX05 BN of 2.5 mm thickness is introduced into each cavity.
• the matrix as represented in FIG. 2 is a sectional view along the line AA of FIG.
• Plates 9 made of graphite make it possible to conduct the current and to press the pistons in a homogeneous manner.
• these plates 9 each comprise four cylindrical cavities 10 machined in one of their faces. These cavities 10 here have a diameter of 20.5 mm and a depth of 1 mm. They allow the pistons to be anchored there and ensure the maintenance in place of the trays,
• a high-pressure uniaxial compact is then made at a threshold close to the maximum density achievable in green by the pulverulent material. In this case, a pressure of 50 MPa is applied for 1 minute.
• a felt 8 made of thermally insulating material for example a graphite felt, having the dimensions of a matrix, is cut and placed above, below and around the matrix in order to limit thermal radiation during heating. SPS fryer.
• FIG. 3 shows an example of SPS 100 sintering device for implementing the device according to the invention, this device including in particular an enclosure 11 and means 12 for applying a current and a load (or pressure). to compression matrix 1.
• the matrix as shown in Figure 3 has only two cavities and Papyex TM sheets and discs, as well as the discs of boron nitride n have not been represented.
• the SPS sintering cycle is then carried out by simultaneously applying a pressure and a pulsed electric current to raise the temperature of the compacts to a bearing temperature sufficient to cause sintering of the compact powder.
• its duration during which its bearing temperature is maintained is relatively short and is generally between a few seconds to a few minutes (generally less than 10 minutes).
• the temperature, the pressure and the time are optimized to obtain the desired density.
• the SPS sintering was carried out at a pressure of 20 MPa per piston, with a rise in temperature of 50 ° C./min, a plateau temperature of 2000 ° C. for a duration of 2 minutes, by applying a maximum current of 5540 A.
• the pressure and the temperature are then lowered and the four tablets of compact sintered are extracted from the matrix.
• its surface of the pellets is ground to remove traces of Papyex TM and residual BN, for example by polishing with abrasive paper, using a diamond disc or a reiffer equipped with a diamond tool.
• the four pellets thus obtained have a mean relative density measured by hydrostatic weighing of 99.33% with a standard deviation of 0.03%.
• the method which is the subject of the invention therefore makes it possible to increase the relative density of B 4 C after sintering compared with the Hot Pressing process under similar temperature and pressure conditions. It is recalled that the relative density obtainable by the HP process is of the order of 96%.
• an SPS sintering cycle is by definition very short, it causes only a small grain magnification compared to an HP sintering cycle.
• the process also makes it possible to increase productivity over conventional SPS sintering due to the use of multi-cavity tooling and short sintering cycles.

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• Engineering & Computer Science (AREA)
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• Manufacturing & Machinery (AREA)
• Mechanical Engineering (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
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• 2017
  • 2017-06-30 FR FR1756103A patent/FR3068272B1/fr active Active
• 2018
  • 2018-06-28 JP JP2019572205A patent/JP2020525392A/ja active Pending
  • 2018-06-28 EP EP18762346.7A patent/EP3625192A1/de not_active Withdrawn
  • 2018-06-28 US US16/626,808 patent/US20200115236A1/en not_active Abandoned
  • 2018-06-28 WO PCT/FR2018/051591 patent/WO2019002777A1/fr unknown

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