WO2018167397A1

WO2018167397A1 - Equipment for baking ceramic preforms requiring a high precision

Info

Publication number: WO2018167397A1
Application number: PCT/FR2018/050520
Authority: WO
Inventors: Pauline CHANIN LAMBERT; Sébastien SAUNIER
Original assignee: Association Pour La Recherche Et Le Développement Des Méthodes Et Processus Industriels - Armines
Priority date: 2017-03-17
Filing date: 2018-03-07
Publication date: 2018-09-20
Also published as: FR3064001A1; FR3064001B1

Abstract

The invention relates to a piece of equipment for sintering bulk parts (12) made of ceramic comprising a microwave cavity (1) and a crucible (11) associated with a crucible holder made of porous refractory ceramic and a susceptor (17, 27) that produces infrared radiation under the action of the microwave radiation. a) Said cavity is a multimode microwave cavity coupled via a waveguide to a microwave generator that operates at a frequency of 2.45 GHz. b) Said holder is made up of two independent parts made of porous refractory ceramic comprising: - a base (13) - at least two axisymmetrical coaxially interlockable rings (14, 15), c) Said crucible (11) has a tubular jacket closed by a bottom covered with millimetre-sized spheres, said crucible (11) being able to be placed in the interior of the interior ring (15), d) Said susceptor material (11) is placed between the wall of said interior ring (15) and the wall of said

Description

CERAMIC PREFORMING COOKING EQUIPMENT NECESSITANT

GREAT PRECISION

Field of the invention

The present invention relates to the field of technical ceramics and more particularly to ceramic preforms baking equipment, optionally pre-sintered, by a heat treatment obtained by microwave electromagnetic radiation. Microwave cooking is obtained by the radiation of an electromagnetic wave leading to the creation of dipoles in certain materials. These dipoles try to follow the movement induced by the microwave. Then there is a phase shift in the dipole response. It is this phase shift which leads to the heating of the material, in the core of the material subjected to radiation. This heating makes it possible to reach the high temperatures required for sintering: the sintering temperature of the ceramic pieces is very high (for example of the order of 1500 ° C. for zirconia prostheses).

The field of the invention relates more particularly to cooking equipment comprising a microwave cavity in which the preform is placed, generally with a susceptor interacting with the microwave radiation from room temperature, the preform allowing a volume heating when its temperature exceeds a value of the order of 700 ° C.

These devices are in particular intended to produce products of relatively small volume and shape that can be relatively complex, for example medical products such as dental prostheses, prostheses or bone substitutes or automotive parts, aeronautical or space or more parts industrial, jewelery or decorative. These examples are of course not limiting.

State of the art

The general principle of sintering technical ceramics in a microwave cavity is well known and appreciated for the possibility of drastically reducing the treatment time, compared to a thermal oven.

For example, it is known from international patent application WO 2016156275 describing a method of heat treatment of surface coating on a metal part by microwaves. This method of treating a surface coating of a solid metal part, comprising the steps of placing in a cavity at least one said metal part having a surface coating capable of absorbing microwave at the frequency w _0, surrounded one or a plurality of first susceptors whose dimensions, the material and the arrangement are configured to screen said microwaves at the frequency v0, in the vicinity of each said metal part, and to emit said microwaves at the frequency v ₀ in said cavity.

The crucible components have rectangular sections. The only annular piece is a removable metal receptacle whose function is to homogenize the heating by moving the sample.

Also known is Japanese Patent Application JP2007119267 describing a method for producing a cooked body in a microwave oven. Lime containing carbon are arranged in absorbent plates having a rectangular section.

Patent Application EP2105691 describes another example of a furnace formed by a box of rectangular section, a heating device such as inductive heating or a heating device. microwave, and an optical temperature determining element for determining the temperature in a combustion chamber. A dental specimen is supported on and / or in and / or below and / or adjacent to cuboid shaped combustion aids which are provided in the combustion chamber. The optical temperature determining element determines the temperature of the combustion aids and / or the combustion object, and is formed by a pyrometer. An independent claim is also included for a heat treatment method, sintering, of a dental combustion object. Balls are arranged to avoid deformation of the object to be sintered

Disadvantages of the Prior Art For the production of technical ceramic parts requiring high precision, such as dental prostheses, and having both angular surface states and small but nevertheless significant volumes, such as a bridge, the solutions of the prior art are not quite suitable.

Indeed, some solutions of the prior art favor the penetration of microwaves in depth, and recommend in this case the use of relatively low frequencies, for example 915 MHz. To increase the energy applied to the part and to achieve the temperatures required for sintering, these solutions recommend using a single-mode cavity.

In other solutions, a susceptor, for example a semiconductor material, is associated with the crucible. This susceptor is constituted by a room configured to emit infrared radiation during interaction with microwaves. This solution, well known in the prior art, makes it possible to start the climb in temperature from room temperature, where the technical ceramic piece to sinter interacts only very poorly with microwaves. Of course, the use of such susceptors requires the provision of thermal insulation to manipulate the crucible, for example at the time of removal of the cavity.

This insulation is provided in the prior art by a ceramic housing transparent to microwaves. However, when the temperatures required for sintering are reached, of the order of 1500 ° C., this housing itself undergoes very strong stresses, equivalent to that of the workpiece, and therefore the insulators of the prior art are extremely fragile. The rupture in the cavity has a very detrimental effect because the crucible can tilt, the housing to degrade by a change in its geometry which induces significant heat losses, limiting the maximum temperature achievable and seriously disrupts the sintering process, or make difficult the removal of the cavity. In any case, the fragility of the supports of the prior art generally limits them to a single use, for a single sintering cycle.

Solution provided by the invention

In order to overcome these drawbacks, the invention relates, according to its most general meaning, to an equipment for sintering solid pre-sintered ceramic parts comprising a microwave cavity and a crucible associated with a porous refractory ceramic crucible support and a susceptor producing infrared radiation under the action of microwave radiation, characterized in that:

a) said microwave cavity is a multimode microwave cavity coupled by a waveguide to a microwave generator operating at a frequency of 2.45 GHz, b) said crucible holder is constituted by independent pieces of porous refractory ceramic comprising:

- a pedestal

at least two coaxially interlocking rings, while preserving an annular interspace of axisymmetric shape open at both ends, the thickness of their walls being constant, the walls having a radius of curvature greater than 10 millimeters at any point of the circumference, said rings being able to rest on said base,

c) said crucible having a tubular envelope closed by a bottom covered with millimetric balls, said crucible being able to be disposed inside the inner ring,

d) said susceptor material being disposed between the wall of said inner ring and the wall of said crucible.

The specific shape of the interlocking rings, asymmetric and without sharp angles, with a constant thickness, makes it possible to avoid the risks of rupture.

The equipment advantageously has one or more of the following additional features:

said rings have an open cylindrical geometry with a wall thickness of between 3 to 10 mm;

said rings are made of porous refractory aluminosilicate;

the thickness of said rings is advantageously from 10 to 20 millimeters, and preferably from 15 millimeters;

said rings are made of porous refractory ceramic resistant to temperatures above 1300 ° C; said base is a solid base with a thickness of between 10 and 50 millimeters;

the support further comprises a riser disposed between the base and the bottom of the crucible;

said riser is constituted by an aluminosilicate disk;

said riser is constituted by a disc made of silicon carbide;

the riser is constituted by a stack of aluminosilicate elements;

it comprises a superposition of an aluminosilicate riser and a silicon carbide extension;

the susceptors are made of a refractory oxide material, chromite lantane, silicon carbide, lantane chromic oxide, refractory ceramic, carbonitride, transition metal, carbide ceramic oxide or carbosilicone;

the susceptors consist of at least one ring of annular shape;

- The susceptors are constituted by bars that surround the crucible;

said millimeter balls are made of sintered zirconia, unsintered alumina or sintered alumina;

- Said cavity is movable relative to the receiving plate of the crucible holder between a position where it forms a closed volume and a position where it is spaced from said plate.

The balls are distributed and arranged to allow the object to sinter movements and deformations, including expansion.

The invention also relates to a crucible support for sintering solid pre-sintered ceramic parts, characterized in that it consists of independent porous refractory ceramic parts comprising: - a pedestal

at least two coaxially interlocking rings while preserving an annular interspace of axisymmetric shape open at both ends, the thickness of their walls being constant, the walls having a radius of curvature greater than 10 millimeters at any point of the circumference, said rings being able to rest on said base.

Detailed description of a non-limiting example of

production

The present invention will be better understood on reading the description which follows, refer to non-limiting examples of embodiment illustrated by the accompanying drawings in which:

FIG. 1 represents a schematic sectional view of an equipment according to the invention

FIG. 2 represents a schematic perspective view of a crucible holder and crucible

FIG. 3 represents a view from above of an assembly formed by the support and the crucible

FIG. 4 represents the variation of the temperature of the preform as a function of time, during a treatment cycle according to

The invention.

Description of an example of equipment

The equipment according to the invention comprises a multimode microwave cavity (1) coupled in known manner to a generator with magnetron via a waveguide. The magnetron provides power from 600 W to 6 kW at a frequency of 2.45 GHz. At the end of the waveguide, pistons make it possible to modify the distribution of the electromagnetic field inside the cavity.

The multimode cavity (1) has dimensions 430 x 430 x 490 mm. It opens with an access hatch (2) equipped with a transparent porthole (3).

It comprises a fixed plate (4) formed by a crushed alumina insulation.

To homogenize the electromagnetic radiation, the cavity (1) also comprises a not shown copper wave stirrer.

The cavity (1) also has a first chimney (5) for observation by a camera (6) of the sample, as well as chimneys (7, 8) for sighting by two infrared pyrometers (9, 10).

Detailed description of the crucible holder

The crucible (11) containing the sample (12) is placed in a porous refractory ceramic carrier transparent to electromagnetic radiation at least at low temperatures of less than 700 ° C. It is made of dense alumina.

This support shown in perspective in Figure 2 and in top view in Figure 3 is constituted by a free assembly of porous refractory ceramic parts without bonding or mechanical connection. It comprises a base (13) formed by a 40-millimeter thick disc and a 100-millimeter section. It is for example made of aluminosilicates. This base (13) provides insulation with respect to the fixed plate (4) and serves as a support for receiving two rings (14, 15) of circular or oval sections. The spacer space (21) between the walls of the rings (14, 15) constitutes a layer of separation and thermal insulation air. By way of example, the section of the outer ring (14) is 100 millimeters and the thickness of its wall 4 millimeters.

The section of the inner ring (15) is 80 millimeters and the thickness of its wall 4 millimeters.

The section of the crucible (11) is 60 millimeters.

In the example illustrated in FIG. 1, the crucible rests on an intermediate disk (16) shifting the bottom of the crucible (11) with respect to the interface between the rings (14, 15), in order to limit direct thermal radiation. .

In the example described in FIG. 1, the crucible is surrounded by a ring made of a susceptor material (17). In the examples illustrated in FIGS. 2 and 3, the susceptors consist of bars made of a material such as Perovskite LaCr0 ₃ oxide or molybdenum silicide (Molybdenum disilicide) MoSi ₂ (17) and bars made of a material such as silicon carbide (27) disposed between the outer wall of the crucible (11) and the inner wall of the inner ring (15).

The susceptor elements Sic, LaCr0 ₃ or MoSi2 can initiate heating.

The bottom of the crucible (11) is covered by millimetric balls, for example zirconium, alumina or in the same material as the part (12) to be treated. This carpet of marbles fulfills two functions:

- From a temperature of the order of 700 ° C, they themselves become thermally radiating and contribute to the heating of the workpiece (12); the balls made of ZrO 2 and / or Al ₂ O ₃ allow a faster temperature rise.

- They also provide a sliding surface of the workpiece (12) whose volume slightly changes shrinkage during the sintering treatment, with a volume variation typically of the order of 20%. The assembly is closed by a disc cover (19) which rests on the end faces of the two rings (14, 15). This lid has a section of about 100 millimeters and a thickness of 10 millimeters. It has a central window (20) with a section of about 20 to 65 millimeters to allow observation by the camera (6) and the pyrometers (9, 10).

Process for manufacturing a technical ceramic part

according to the invention

The manufacture of a technical ceramic part comprises a known step of preparing a pre-sintered preform, and a sintering step in equipment according to the invention.

The preparation step of the preform comprises the following tasks:

Preparation of the powders: formulation, atomization, diffusion and dispersion of the ceramic powders by mixing and grinding. This task makes it possible to improve the particle size distribution and to homogenize the density of the green material.

Formatting: this process aims at the consolidation of the powders and the desired shaping and the state, dry or wet, of the material according to shaping techniques: pressing, extrusion, injection, casting ... This shaping takes into account the shrinkage resulting from sintering. The preform therefore has dimensions larger than the expected shape. These dimensions are determined by applying a homothetic shrinkage model to a three-dimensional digital representation of the final shape. Heat treatment: the preform is then subjected to different treatments such as debinding, decarbonation, removal of additional water if necessary. Sintering causes physical transformations which can modify the nature of the phases in the presence and the microstructure of the material, which makes it possible to densify the powders and obtain the desired phase. Pre-sintering is a special phenomenon due to the heat treatment at intermediate temperatures of less than 1000 ° C.

The preform is then introduced into the crucible (11), which is itself introduced into the support closed by the lid, the assembly then being placed in the microwave cavity.

Power is then applied for a slope of about 100 ° C per minute to reach a temperature of about 700 ° C in about 7 minutes and 1550 ° C in 20 minutes. Up to the temperature of about 700 ° C, the heating is essentially surface, provided by convection susceptors that emit infrared radiation by interaction with microwave radiation.

Beyond this temperature, the ceramic part and the balls on which it rests interact themselves with the electromagnetic radiation which increases the heating, with a predominantly voluminal heating.

The piece is then held at a temperature of about 1500 to 1575 ° C for a period of 10 to 20 minutes. Then the magnetron is stopped and the temperature decreases in a few minutes; which makes it possible to remove the support and the crucible from the cavity.

FIG. 3 represents an example of a curve of evolution of the temperature of a preform as a function of time. A first slope of elevation of the temperature is observed up to about 700 ° C, then an acceleration due to the fact that at this temperature the preform and the millimeter balls begin to interact themselves with the microwave radiation to contribute with the mass heating and by thermal radiation of the balls millimeters. The temperature is then maintained in plateau.

Claims

claims

1 - Equipment for sintering ceramic solid pieces (12) comprising a microwave cavity (1) and a crucible (11) associated with a porous refractory ceramic crucible holder and a susceptor (17, 27) producing an infrared radiation under the action of microwave radiation characterized in that:

a) said microwave cavity (1) is a multimode microwave cavity coupled by a waveguide to a microwave generator operating at a frequency of 2.45 GHz,

b) said crucible holder is constituted by independent pieces of porous refractory ceramic comprising:

- a base (13)

- At least two rings (14, 15) nestable coaxially maintaining an annular spacer space (21) of axisymmetric shape open at both ends, the thickness of their walls being constant, the walls having a radius of curvature greater than 10 millimeters at any point of the circumference, said rings (14, 15) being able to rest on said base (13),

c) said crucible (11) having a tubular envelope closed by a bottom covered with millimetric balls, said crucible (11) being adapted to be arranged inside the inner ring (15),

d) said susceptor material (11) being disposed between the wall of said inner ring (15) and the wall of said crucible (11).

2 - Equipment for sintering massive pieces of pre-sintered ceramic according to claim 1 characterized in that said rings (14, 15) have a geometry cylindrical open with a wall thickness of between 3 to 10 mm.

3 - Equipment for sintering massive pre-sintered ceramic parts according to claim 1 characterized in that said rings (14, 15) are made of porous refractory aluminate.

4 - Equipment for sintering massive pieces of pre-sintered ceramic according to claim 1 characterized in that said rings (14, 15) are made of porous refractory ceramic resistant to temperatures above 1300 ° C. 5 - Equipment for the sintering massive pre-sintered ceramic parts according to claim 1 characterized in that said base (13) is a solid base with a thickness of between 10 and 50 millimeters. 6 - Equipment for sintering solid pre-sintered ceramic parts according to claim 1 characterized in that it further comprises a riser (16) disposed between the base (13) and the bottom of the crucible (11). 7 - Equipment for sintering massive pre-sintered ceramic parts according to the preceding claim characterized in that said riser (16) is constituted by an aluminosilicate disk. 8 - Equipment for sintering massive pre-sintered ceramic parts according to the preceding claim characterized in that it comprises a superposition of an aluminosilicate riser and a silicon carbide extension. 9 - Equipment for sintering massive pre-sintered ceramic parts according to claim 6 characterized in that said riser (16) is constituted by a silicon carbide disk.

10 - Equipment for sintering solid pre-sintered ceramic parts according to claim 1 characterized in that the susceptors (17, 27) are made of a refractory oxide material, chromite lantane, silicon carbide, oxide of chromium of lantane or carbosilicone.

11 - Equipment for sintering massive pieces of pre-sintered ceramic according to claim 1 characterized in that the susceptors (17, 27) are constituted by at least one annular ring.

12 - Equipment for sintering solid pre-sintered ceramic parts according to claim 1 characterized in that the susceptors (17, 27) are constituted by strips which surrounds the crucible (11).

13 - Equipment for sintering solid pre-sintered ceramic parts according to claim 1 characterized in that said millimeter balls (18) are made of sintered zirconia, unsintered alumina or sintered alumina.

14 - Equipment for sintering solid pre-sintered ceramic parts according to claim 1 characterized in that said cavity (1) is movable relative to the plate (4) for receiving the crucible holder between a position where it forms a volume closed and a position where it is spaced from said plate (4). Crucible holder for sintering massive pieces of pre-sintered ceramic, characterized in that it consists of independent porous refractory ceramic parts comprising:

- a base (13)

- At least two rings (14, 15) nestable coaxially maintaining an annular spacer space (21) of axisymmetric shape open at both ends, the thickness of their walls being constant, the walls having a radius of curvature greater than 10 millimeters at any point of the circumference, said rings (14, 15) being able to rest on said base (13).