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
• • 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
  • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
  • C04B35/10—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on aluminium oxide
• • 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
  • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
  • C04B35/48—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on zirconium or hafnium oxides, zirconates, zircon or hafnates
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/64—Heating using microwaves
  • H05B6/647—Aspects related to microwave heating combined with other heating techniques
  • H05B6/6491—Aspects related to microwave heating combined with other heating techniques combined with the use of susceptors
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/64—Heating using microwaves
  • H05B6/80—Apparatus for specific applications
• • 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
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
  • C04B2235/3217—Aluminum oxide or oxide forming salts thereof, e.g. bauxite, alpha-alumina
• • 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
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
  • C04B2235/3231—Refractory metal oxides, their mixed metal oxides, or oxide-forming salts thereof
  • C04B2235/3244—Zirconium oxides, zirconates, hafnium oxides, hafnates, or oxide-forming salts thereof
• • 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
• • 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
  • C04B2235/667—Sintering using wave energy, e.g. microwave sintering

Definitions

• the invention relates to a method of heat treatment of ceramic materials and more particularly to a method of densifying a piece of ceramic material in a microwave cavity.
• the ceramic material parts can be manufactured by heat treatment to be consolidated and / or densified.
• a solid piece of powder previously shaped, for example by compression or casting, can be densified by heating, or sintering.
• This operation is conventionally performed by heating a powder sample compressed by infrared radiation and / or by convection.
• the heat source emitting infrared radiation is typically performed by a resistive element or by the combustion of a gas.
• the sample is typically heated to a temperature above 700 ° C.
• the efficiency of heat treatment by this type of method is not optimal and leads to significant energy losses, additional production costs and a major environmental impact. In the case of gas ovens, heating results in carbonaceous gas emissions harmful to the environment.
• Microwave ovens offer an interesting alternative to these two methods of heat treatment. To heat non-metallic materials, their energy efficiency is much higher than that of the two methods described above, which can lead to a significant saving of the energy used in the case of convection ovens. This efficiency can be explained by an absorption of localized energy within the sample and by the reduction of the total volume to be heated. Microwave ovens also reduce heat treatment time compared to conventional methods.
• the heating of pieces of ceramic material of large dimensions is little or not compatible with microwave heating.
• the dielectric properties of many ceramic materials are not conducive to coupling with microwaves at room temperature, up to temperatures typically of the order of 400 ° C.
• the dielectric losses of zirconia increase significantly above 400 ° C, resulting in a better coupling between zirconia and microwaves above this temperature.
• the heatable thickness of a sample (substantially corresponding to the length of penetration into the sample of microwaves) is dependent on both the characteristics of the material but also on the frequency v 0 of the microwaves. waves: the length of penetration increases when the frequency decreases.
• the size of a piece of ceramic material heated by the energy of microwaves dissipated in said room is in this case limited.
• a monomode cavity makes it possible to thermally treat a sample in a homogeneous manner in a volume of the cavity: this volume is smaller as the frequency of the microwaves introduced into the cavity is high.
• a typical single-mode cavity in which microwaves are emitted at a frequency of 2.45 GHz makes it possible to treat a sample with a volume typically less than 0.35 L.
• a solution of the prior art consists in using a lower frequency v 0 equal to 915 MHz.
• S. Li et al. Li, S., Xie, G., Louzguine-Luzgin, DV, Sato, M., & Inoue, A. (2011) Microwave-induced sintering of Cu-based metallic composite glass matrix in a single-mode 915-
• these applications apply this solution to the heat treatment of an amorphous metal alloy, different from a ceramic material.
• the temperature of the heat treatment is 400 ° C.
• the maximum heat treatment temperature is limited by the appearance of plasma and / or electric arc, driven by an intense electromagnetic field.
• Sintering a ceramic material requires processing of samples at elevated temperatures, for example between 1300 ° C and 1600 ° C. It is problematic to reach these temperatures during a heating Microwave: A high intensity of the electromagnetic field is typically required. When the sample or any other part inside a microwave cavity can reflect the microwaves (even partially), an intensity locally increased by microwave reflection may cause the appearance of a plasma. The appearance of a plasma has a dramatic effect on the heat treatment of a sample. Plasmas comprise charged particles free in volume and therefore highly conductive: a plasma has the characteristic of reflecting an incident electromagnetic field. This plasma can cause a major disruption of heating to cause a rapid and significant drop in the temperature of the sample. The appearance of a plasma also causes a disturbance of the spatial distribution of the electromagnetic field in a cavity and consequently a heterogeneous heat treatment of the treated part (s).
• Another solution is to use, in a monomode oven at 915 MHz, two parallel susceptors, whose surface is perpendicular to the electric field present in the cavity (R. Heuguet, "Development of single-mode microwave processes at 2450 MHz and 915 MHz for the sintering of oxide ceramics ", Thesis defended on October 14, 2014, University of Caen Basse Normandie), the two susceptors surrounding the sample to be heat treated. Indeed, being perpendicular to the electric field, the susceptors cause an electric field concentration on the sample. This makes it possible to minimize the necessary microwave power and thus to greatly limit the creation of plasma in the vicinity of the sample. This solution makes it possible to reach temperatures of the order of 1500 ° C. The present inventors have however realized that, when high microwave powers are required, the appearance of a plasma, in the vicinity of the susceptors, is observed which then impairs the process.
• the invention aims to remedy some or all of the aforementioned drawbacks of the prior art, and more particularly to heat treat, at least in part by microwaves, a piece of ceramic material with a volume greater than 1 cm 3 , and in the case of a piece of porous ceramic material, to densify it in measurements equivalent to a densification carried out by methods of the prior art using for example convection ovens.
• An object of the invention making it possible to achieve this object is a method of heat treatment of at least one solid piece of ceramic material in a microwave cavity, said cavity being formed by an enclosure whose geometry is adapted to the resonance in a single mode of an electromagnetic field defining at least one local extremum of the electric or magnetic field in said cavity, at a frequency v 0 between 900 MHz and 1 GHz, the direction of the electric field E being substantially uniform in said empty cavity , comprising at least the steps of:
• a said solid part is initially porous and densified at least one said solid part by heating in step b).
• At least two said solid pieces are brazed in step b).
• at least one element chosen from an edge and an apex of at least one said first susceptor is rounded.
• At least one said first susceptor is silicon carbide.
• the material of at least one said ceramic part is chosen from alumina and zirconia.
• at least one said piece of solid ceramic material is densified so as to comprise at least 90% ceramic material per unit volume.
• said method comprises a step of placing said one or more susceptors and said one or more pieces of ceramic material in a first thermal confinement.
• said first thermal confinement is surrounded by one or a plurality of second susceptors.
• said arrangement of said second susceptor (s) forms a second volume delimited by said second susceptor (s) and in which the dimensions, the material and the arrangement of said second susceptors are configured to emit infrared radiation during an interaction with microphones. -ondes.
• said one or more second susceptors and said first thermal confinement are arranged in a second thermal confinement.
• each said second susceptor comprises at least a second main surface, each said second main surface being a regulated surface whose generatrices are parallel to said electric field E in a said empty cavity.
• at least one element chosen from an edge and an apex of at least one said second susceptor is rounded.
• the material of at least one said susceptor is selected from a refractory and semiconductor oxide of a transition metal, and a carbide.
• the material of said one or more first and second susceptors is selected from silicon carbide and lanthanum chromite.
• said ceramic material comprises several phases of different ceramic materials and the dimensions, the material and the arrangement of each said first susceptor are configured to heat-treat selectively at least one of said phases of each said piece of ceramic material.
• the maximum size D of said part is chosen so that the ratio between the penetration length of said microwaves in said part and D is between 0.5 and 10.
• FIG. 1 schematically illustrates the section a device used for the implementation of the invention
• Figure 2 is a photograph of a part of a device used for the implementation of the invention
• FIG. 3 is a diagrammatic representation of a side view of the cavity, comprising a part, and electric and magnetic fields associated with different configurations of the cavity;
• Figure 4 is a schematic representation of an indirect heating method, different from the invention;
• Figure 5 is a schematic representation of a direct heating method, different from the invention;
• Figure 6 is a schematic representation of a hybrid heating method according to one embodiment of the invention;
• Figure 7 is an illustration of a simulation of the intensity of the electric field around a susceptor different from a susceptor implemented in the invention;
• Figure 8 is a set of simulation illustrations of the intensity of the electric field around a susceptor different from a susceptor implemented in the invention;
• FIG. 9 illustrates the kinetics of the temperature of a piece of ceramic material during a heat treatment according to one embodiment of the invention;
• FIG. 10 is a scanning electron micrograph of a sectional cut of ceramic material after heat treatment according to one embodiment of the invention.
• Figure 1 schematically illustrates the section of a device used for the implementation of the invention.
• microwaves are considered to be electromagnetic waves whose frequency is between 300 MHz and 300 GHz.
• the frequency of the microwaves 1 used in the invention is between 900 MHz and 1000 MHz, so as to partially meet the problems of the prior art: one chooses a microwave frequency among the lowest frequencies of the range microwaves frequencies so as to heat a solid piece of ceramic material 4 with the greatest penetration length possible, and so as to have a volume able to heat a room homogeneously in a microwave cavity the largest possible.
• a monomode resonant cavity illustrated schematically in FIG. 1, comprises a volume of 9 liters able to heat a sample homogeneously when the frequency of the microwaves 1 is 915 MHz.
• the microwaves 1 are for example emitted into the cavity in a direction normal to the plane of the section illustrated in FIG. 1.
• a similar cavity but adapted (for example geometrically) to be monomode resonant for microwaves 1 of frequency equal to 2.45 GHz, would have a similar volume 25 times smaller.
• the size of the solid piece of ceramic material 4 smaller than the size of the cavity.
• D is the maximum size of a part 4
• the method is carried out in a cavity 9, formed by an enclosure, whose geometry is adapted to propagation, as well as to resonance in a single mode (single mode ) of the electromagnetic field, at a frequency v 0 between 900 MHz and 1 GHz, preferably substantially equal to 915 MHz.
• a single mode single mode
• the cavity 9 is then called monomode.
• the geometry of the cavity 9 can be adjusted before the introduction of a sample so as to be monomode.
• the cavity can be adapted by varying, for example, the parameters of a movable short-circuit piston or of an iris in waveguides introducing microwaves 1 into the cavity. the cavity.
• the electric field E in the empty cavity when microwaves 1 are emitted, has a uniform direction.
• the direction of the field E is uniform in the volume in which a solid piece of ceramic material 4 is placed during a heat treatment process, and advantageously a densification process.
• An illustrative vector E is illustrated in FIG.
• At least one solid piece of ceramic material 4 is placed in a cavity 9. It is advantageously placed on a support made of thermal insulation 7.
• solid piece of ceramic material is meant a part, comprising at least one ceramic material, adapted to support itself mechanically, for example placed on a support, in opposition to a powder of ceramic material disposed in a crucible.
• a solid piece of ceramic material 4 may be porous.
• porous is meant that a solid piece 4 comprises pores, that is to say volumes capable of containing a gaseous or liquid medium.
• a porous material is characterized by a material whose ratio between the pore volume and the apparent volume of the material is substantially different from zero, preferably greater than 1%.
• the solid part 4 can support itself, placed on a support, thanks for example to the connections between different grains of the material ensuring a mechanical stability of the part.
• the ceramic material of a solid piece of ceramic material 4 is adapted to absorb microwaves 1 at the frequency v 0 and at a temperature T greater than or equal to 700 ° C.
• the material of a solid piece 4 may be a ceramic oxide, for example chosen from alumina, zirconia and spinel.
• the propagation mode of the microwaves 1 in the cavity 9 can be chosen so as to optimize the absorption of the microwaves 1 by the material of the part 4.
• At least one Local extremum of electric field and / or stationary magnetic can be formed at distinct locations in a single-mode cavity 9.
• bellies and nodes of the electric and / or magnetic field may be arranged longitudinally in a cavity 9 and in quadrature phase.
• a solid piece 4 made of ceramic material is arranged in correspondence with an electric or magnetic field belly in the cavity 9.
• the thermal insulator 7 may be, for example, the thermal insulator 7 liteCell (AET Technologies, thermal insulation with high alumina content).
• the solid piece of ceramic material 4 is surrounded by at least a first susceptor 3.
• a solid piece of ceramic material 4 is surrounded by two first susceptors 3, respectively on the left and to the right of the solid piece of ceramic material 4.
• one or a plurality of first susceptors 3 may surround a solid piece of ceramic material 4.
• at least one element selected from a ridge and a peak of at least one said first susceptor is rounded. This feature limits or prevents the appearance of plasma during heat treatment.
• rounded it is defined that the different walls of a first susceptor 3 meet in edges and / or vertices whose surface follows at least one radius of curvature of which the length is greater than one thousandth of the maximum dimension of the cavity 9 and preferably to one hundredth of the maximum dimension of the cavity 9.
• the dimensions, the material and the arrangement of the first susceptor (s) 3 are chosen, or configured to emit infrared radiation directly to a said solid part 4 during an interaction with microwaves (1) at the frequency v 0 , at the neighborhood of each said solid piece 4 or around each said piece 4.
• directly it is defined that the trajectory of an infrared radiation, emitted by one or more first susceptors 3, to the solid piece or pieces 4, does not cross no other piece of solid material and propagates only in the gaseous phase surrounding the solid part (s) 4.
• Neighborhood means a length, less than the characteristic length of one or a plurality of solid parts made of ceramic material 4.
• a susceptor is a material capable of excellent radiation absorption of microwaves 1 at a given frequency. Upon absorption of this radiation, the susceptor material can re-emit the energy absorbed by infrared radiation 2 for example.
• the absorption of a susceptor material is governed by high dielectric, electrical or magnetic losses during the excitation of the material by an electromagnetic field, as for example in the case of microwaves 1.
• the materials used as first and / or or second susceptors in the embodiments of the invention may advantageously be silicon carbide (SiC) and / or lanthanum chromite (LaCr0 3 ). Other materials with high microwave absorption capabilities can be used. Materials comprising a refractory and semiconductor oxide of a transition metal may be used. It is also possible to use materials composed of carbides, such as boron carbide, for example.
• the first susceptors 3 comprise at least a first main surface 5.
• main surface is meant that the arrangement of a part or the whole of a first susceptor 3 or a second susceptor 12 may be defined by a surface.
• a main surface may be a plane: FIG. 1 illustrates, for example, two first susceptors 3 whose first major surfaces 5 are planes seen in section. One of them is illustrated with white dots.
• a main surface may also be curved, for example in the case of side surface of a cylinder.
• each said first or second main surface 5, 21 of each said first or second susceptor 3, 12 is a regulated surface, whose generatrices are parallel to the electric field E of the empty cavity 9, and / or the volume adapted to receive the sample.
• the local absorption of the microwaves 1 makes it possible, according to the arrangement of the various susceptors in the cavity 9, to configure a volume in which the solid part 4 can be heated directly by the first susceptor (s) 3 by infrared radiation.
• the assembly formed by a solid piece of ceramic material 4, and the first susceptor 3 surrounding a solid piece of ceramic material 4, is arranged (or placed) in a first thermal confinement 10 made of thermal insulation 7.
• the thermal insulation 7 may be of liteCell type (AET Technologies SAS, high-alumina heat insulator), and / or Quartzel (trademark, Saint-Gobain Quartz SAS). This confinement by a thermal insulator 7 makes it possible to limit the energy losses by radiation during the heat treatment.
• the form of thermal confinement 10 may be cylindrical. In the embodiment of the invention presented in FIG. 1, two second susceptors 12 surround a first thermal confinement 10.
• the assembly composed of the second susceptors 12 and the first thermal confinement 10 is surrounded by a second thermal confinement 11, made by isolating This structure makes it possible to increase the thermal confinement properties.
• the second thermal confinement 11 is produced by a thermal insulator 7
• the second thermal confinement 11 is placed on an aluminum plate 8.
• the cavity 9, the first thermal confinement 10 and the second thermal confinement 11 can be drilled in order to achieve a pyrometric aiming 6.
• This aiming 6 can enable a temperature sensor to remotely measure the temperature of a solid piece of material ceramic 4 during heat treatment.
• the temperature sensor and the microwave transmitter 1 are connected via a bus to a processing unit.
• the processing unit comprises one or more microprocessors and a memory.
• the processing unit independently controls the transmit power of the microwave transmitter and processes the information of the temperature sensor.
• the power is slaved to a given temperature setpoint.
• the temperature setpoint can be variable in time so as to produce defined temperature treatment profiles, such as temperature ramps or stationary temperature heat treatments. According to one embodiment of the invention, it is possible to measure, during all or part of the emission of microwaves 1, the temperature of a solid piece of ceramic material 4 and then to regulate or control the power of emission of microwaves according to the measured temperature.
• FIG. 2 is a photograph of a part of a device used for the implementation of a method of the invention.
• a solid piece of ceramic material 4 is schematically illustrated by a white rectangle for the clarity of the photograph.
• Two first susceptors 3 surround the solid piece of ceramic material 4.
• Surround is meant here that at least half of the surface of a solid piece of ceramic material 4 is in the vicinity of a first susceptor 3.
• the first main surface 5 of one of the first susceptors 3 is represented by a white dashed rectangle in perspective.
• Field E is shown at the bottom right of the photograph.
• the first major surfaces of the two susceptors, planar are parallel to the direction of the field E.
• the first susceptors 3 and the solid piece of ceramic material 4 are placed inside a first thermal confinement 10, formed in part by the four bricks illustrated by the photograph.
• FIG. 3 is a schematic representation of a side view of the cavity 9, comprising a part 4, and electric and magnetic fields associated with different configurations of the cavity 9.
• a cavity 9 may be formed of walls, a coupling iris 19 at one of its ends and a short circuit piston 20 at the other of its ends.
• a first configuration (a) is associated with a position of a coupling iris 19 and a position of a short circuit piston 20, marked by irregular discontinuous lines.
• a second configuration (b) is associated with another position of a coupling iris 19 and another position of a short circuit piston 20, also marked by irregular discontinuous lines. In the middle of FIG.
• the amplitude of the electric field (c) and the amplitude of the magnetic field (d) are schematically illustrated and correspond to the configuration (a) of the cavity.
• the amplitude of the electric field (c) and the amplitude of the magnetic field (d) are schematically illustrated and correspond to the configuration (b) of the cavity.
• the part 4 is placed in correspondence with a local extremum of electric or magnetic field.
• the piece 4 In the configuration (a) of the cavity, the piece 4 is placed in correspondence with a belly (or extremum), the amplitude of the magnetic field (d) and with a node of the electric field (c).
• the part 4 In the configuration (b) of the cavity, the part 4 is placed in correspondence with a belly (or extremum) of the electric field (c) and with a node of the magnetic field (d).
• Figure 4 is a schematic representation of an indirect heating method, different from the invention.
• Panel A of FIG. 4 is a diagrammatic representation in plan view of the implementation of indirect heating.
• Indirect heating implements at least a first susceptor 3 and a sample 18 surrounded by the first susceptor 3.
• the material constituting the sample 18 to be heated is transparent to microwaves 1 or opaque in the microwave 1.
• transparent is meant a material whose dielectric and / or magnetic losses are substantially zero when the material is subjected to a microwave field 1 at a given frequency.
• a transparent material generally has a very low electrical conductivity. The electrical conductivity of a transparent material may be less than 10 -8 S. m- 1 , preferably less than 10 -10 S. m- 1 and more preferably less than 10 -12 S. m- 1 .
• opaque is meant a material reflecting the radiation of microwaves 1 for a given frequency.
• An opaque material generally has a high electrical conductivity. The electrical conductivity of an opaque material is preferably greater than 10 3 m.sup.- 1 .
• the interaction between the microwaves 1 and the sample 18 does not allow growth to occur.
• the susceptor 3 placed around the sample 18 absorbs the microwaves 1 and emits infrared radiation 2. The sample can then be heated by infrared radiation 2.
• the panel B of FIG. 4 schematically illustrates a temperature profile along an axis passing through the center of the sample 18.
• the two temperature maxima of this implementation are located at the distance, corresponding to the axis d on the abscissa of the location of the first susceptor 3.
• the temperature in the center of the sample is mainly due to heating by infrared radiation 2 of the periphery of the sample and / or convection of the medium surrounding the sample, coupled to a thermal conduction within the sample as explained above.
• This method of heat treatment does not solve the technical problem posed by the prior art: we lose a significant portion of energy efficiency related to heating by microwave 1.
• FIG. 5 is a schematic representation of a direct heating method different from the invention.
• Panel A of FIG. 5 is a schematic representation in plan view of the implementation of direct heating.
• the material constituting the sample 18 to be heated absorbs the microwaves 1 at a given frequency.
• the interaction between the microwaves 1 and the absorbent material of the sample 18 makes it possible to heat the sample.
• the panel B of FIG. 5 schematically illustrates a temperature profile along an axis passing through the center of the sample 18.
• the temperature profile presents a maximum in the center of the sample.
• the profile may be different because it depends in particular on the size of the sample 18, the material of the sample 18, the power and the wavelength of the microwaves 1 emitted.
• the sample 18 is a solid piece of ceramic material 4, it is possible that the material of the room is not able to be heated directly by microwaves 1 at room temperature.
• a porous piece 4 is densified during a high temperature heat treatment process: in the case of certain ceramic materials, if the density of the piece is too high, the penetration volume of the microwaves 1 may be The effectiveness of the heating by microwaves 1 is thus limited, and does not make it possible to reach certain set temperatures, for example temperatures higher than 700 ° C.
• FIG. 6 is a schematic representation, in top view, of a hybrid heating method according to one embodiment of the invention.
• the implementation of this embodiment of the invention comprises a solid piece of ceramic material 4.
• the sample is surrounded by a first susceptor 3.
• the first susceptor 3 absorbs at a given frequency the micro
• the first susceptor 3 emits in this case an infrared radiation 2 which contributes to the temperature treatment of the solid part made of ceramic material 4, in particular during a first phase of growth of the temperature of the solid part 4, in which the material of the solid part 4 is very weakly capable of interacting with the microwaves 1.
• part of the microwaves 1 can be absorbed, at a given frequency, by the solid piece of ceramic material 4.
• FIG. 7 is an illustration of a simulation of the intensity of the electric field around a susceptor different from a susceptor implemented in the invention. The intensity of the electric field is illustrated by the gray levels of the illustration, the maximum intensity of E corresponding to black.
• a first susceptor is a crucible, useful for example for sintering a ceramic material initially in powder form.
• This first susceptor may also contain a solid piece of ceramic material 4 as illustrated in Figure 7.
• the electric field lines E are illustrated by thin black lines. In the absence of first susceptor and solid piece of ceramic material 4, the field lines would be vertical.
• the crucible-shaped susceptor geometry illustrated does not comprise only first major surfaces 5 whose generatrices are parallel to the electric field E of an empty cavity 9. The inventors have discovered that one or the first major surfaces 5 not parallel to the electric field E of an empty cavity 9 are particularly suitable for driving high spatial areas of the electric field, as well as discontinuities of the electric field on the surface. of a first and / or second susceptor, during the emission of microwaves 1.
• zones are particularly suitable for causing the appearance of plasma and / or electric arcs during heat treatment and / or densification of a solid piece of ceramic material 4.
• the inventors have discovered that it is possible to reduce these areas by placing only one or the first susceptors 3 whose first major surfaces 5 are parallel to the direction of E in an empty cavity, that is to say, each said first main surface 5 is a regulated surface whose generatrices are parallel to E in a cavity 9 empty. It is also possible to reduce these areas by placing in the cavity one or more second susceptors 12, each said second main surface 21 is a regulated surface whose generatrices are parallel to E in a cavity 9 empty.
• Figure 8 is a set of illustrations of simulation of the intensity of the electric field around a susceptor different from a susceptor implemented in the invention.
• the intensity of the electric field is illustrated by the gray levels of the illustration, the maximum intensity of E corresponding to black.
• the panel A of FIG. 8 is a detail of FIG. 7, corresponding to the lower part of the crucible, the geometry of which does not comprise a first main surface 5 parallel to E in an empty cavity.
• the line in dotted line corresponds to the outer surface of the susceptor arranged above the dots.
• the intensity, the variation of the intensity and the discontinuity of the electric field illustrated in the panel A can favor the appearance of plasma and / or electric arc during the emission of microwaves 1.
• the panel B of Figure 8 is a detail of Figure 7, corresponding to a portion to the right of the crucible illustrated in Figure 7.
• This part comprises a first major surface parallel to the field E in a vacuum cavity.
• the average intensity of E is lower than the average intensity shown in panel A.
• the arrangement of this part may allow a temperature rise sufficient for efficient heat treatment and / or densification without forming a plasma and / or an electric arc in the cavity 9.
• FIG. 9 illustrates the kinetics of the temperature of a solid part 4 made of ceramic material during a heat treatment according to one embodiment of the invention.
• the ceramic material used may be alumina.
• the temperature reference is 1600 ° C. This instruction is reached in less than 250 min.
• Three phases of the kinetics can be distinguished: a first phase (between 0 min and about 40 min) during which the slope of the kinetics is on average 9 ° C / min, a second phase (approximately between 40 min and 150 min ) in which the kinetic slope is on average 6.5 ° C / min and a third phase (approximately between 150 min and 210 min) during which the kinetic slope is on average 3.5 ° C .
• FIG. 10 is a scanning electron micrograph of a solid piece section 4 made of ceramic material after a heat treatment according to an embodiment of FIG. the invention.
• the ceramic material used may be alumina.
• the scale bar corresponds to a length of 1 ⁇ .
• the microstructure of the ceramic material of the photograph corresponds to the heat treatment whose kinetics is illustrated in FIG. 9.
• the solid part made of ceramic material 4 used is an alumina-type oxide pellet, the diameter of which is for example 80 mm.
• the measured density of the solid piece of ceramic material 4 is strictly greater than 95% (by volume) and the microstructures observed in the material are fine: in particular, FIG. 10 illustrates a microstructure whose mean diameter of the grains 17 is less than one micrometer and substantially equal to 350 nm.
• the susceptors comprise first major surfaces 5 and / or second major surfaces 21 not parallel to the field E of an empty cavity 9, the appearance of plasma can prevent this target temperature from being reached.
• the heat treatment time, corresponding to a microwave emission step 1, and the power of the microwaves 1 emitted can be parameterized so as to heat treat and / or to densifying a solid piece of ceramic material 4 to a value greater than 90% ceramic material per unit volume.
• the ceramic material of a part 4 can be polyphase, and comprise several phases of different ceramic materials.
• the interaction properties of these materials with microwaves 1 may be different during a microwave transmission 1 of frequency v 0 between 900 MHz and 1 GHz.
• the arrangement of the different first susceptors 3 can make it possible to vary the power dissipated in the different phases and thus to selectively heat treat and / or densify certain or at least one of the phases of a material of a part 4.
• two ceramic pieces 4, porous or not, may be heat treated, so as to be brazed during microwave emission.
• a method according to the invention makes it possible in this case to reach the conventional brazing temperatures of the ceramic pieces by reducing the risks of plasma appearance, by saving energy compared with conventional soldering methods and by reducing the time required. to reach the conventional brazing temperatures (which may be, depending on the ceramic material of a solid part 4, for example between 600 ° C and 1200 ° C).

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• 2016
  • 2016-10-28 FR FR1660496A patent/FR3058138B1/fr active Active
• 2017
  • 2017-10-20 CA CA3041915A patent/CA3041915A1/fr active Pending
  • 2017-10-20 JP JP2019523691A patent/JP7149937B2/ja active Active
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