A process for densifying materials or consolidating an assembly of materials

Abstract

The present invention relates to a method for densifying materials or consolidating a material assembly comprising a single sintering step consisting of the simultaneous application, within a sintering chamber formed by a median zone of a bore in a body (3), of a uniaxial force and a sintering temperature to said material or assembly placed in this sintering chamber, said force being applied by at least two pistons (10, 20) movable towards each other within said bore, said pistons (10, 20) sliding in said bore (1; 2) within said body (3), the assembly formed by said bore (1, 2) and said pistons (10, 20) being sealed. The sintering temperature is produced by the passage of an electric current through said body (3), said body (3) being electrically and thermally conductive. Figure 1

Description

Title of the invention: Method for densifying materials or consolidating an assembly of materials Field of invention

[0001] The present invention relates to the field of industrial production processes for ceramics by sintering metal oxides, for example sapphire powder.

[0002] High-performance ceramics and assemblies are essential materials in a wide variety of fields. Examples include automotive, electronic applications, energy and the environment, mechanical engineering, and medical technology. These materials have exceptional mechanical, electrical, thermal, and biochemical properties.

[0003] Currently, most ceramic materials are produced from powders and sintered at temperatures above 1000°C with the objective of reaching at least 95% of their theoretical densities. The reduction of surface free energy, the driving force of sintering, can be promoted by the application of pressure or by the optimization of diffusion processes using ultra-fast heating routes. Thus, very active research on an international scale has contributed to the development of innovative sintering techniques that have improved the performance of conventional sintering, in particular microwave sintering or pulsed current sintering (PSS). Pressure, by promoting particle rearrangement, plastic deformation and pore shrinkage, plays a beneficial role in sintering, while the high temperatures used in these processes present several technological challenges.These solutions involve heating to high temperatures, which is energy-intensive.

[0004] The use of nanometric powders (with particle sizes between 10 and 100 nm) as starting material has proven to be an effective solution for lowering sintering temperatures because the high surface / volume ratio of nanoparticles facilitates diffusion processes, especially at high temperatures. For example, sintering BaTiO3 requires temperatures of the order of 1300-1400°C starting from micrometric powders, this temperature can be lowered to 800°C.

[0005] To reduce the energy cost, a geo-inspired densification solution, hydro- or solvothermal sintering, has been proposed. The physicochemical mechanisms are of the dissolution-precipitation type via a stress dissolution fining process, the main driving force being the intragrain stress gradients. Under the effect of uniaxial pressure applied to a powder in hydrothermal conditions, stress dissolution creep involves, after dissolution in the interparticle contact zone where the concentration of dissolved species is high, the transport of material via the surrounding liquid phase to the less constrained free surfaces (where the concentration of dissolved species is lower) where precipitation occurs. Given the low temperatures used (generally below 300°C), solid phase diffusion processes cannot be involved. On the other hand, hydrothermal conditions allow for enhanced reactivity at the solid / liquid interfaces. The specificity of hydrothermal sintering is that water acts as both solvent and vector for material transport. In the case of solvothermal sintering, water is replaced by another non-aqueous solvent. This process, also known as Hydrothermal Hot Pressing, was presented in the article Yamasaki, K. Yanagisawa, M. Nishioka, and S.Kanahara, “A hydrothermal hot-pressing method: apparatus and application” J. Mater. Sci. Lett., 5 355-356 (1986). However, relative densities do not generally exceed 80%. Another process called Cold Sintering Process (CSP), derived from hydrothermal sintering, involves densification in an open system where water evaporates during sintering. This process of densifying materials at low temperatures (usually T < 350°C) under hydrothermal conditions, called “reactive Hydrothermal Liquid Phase Densification (rHLPD), the cold sintering process (CSP)” is presented in the article J. Guo, A. L. Baker, H. Guo, M. Lanagan, and C. A. Randall, “Cold Sintering Process: A New Era for Ceramic Packaging and Microwave Device Development”, J. Am. Ceram. Soc., 7 1-7 (2016). . State of the art

[0006] Known in the state of the art is patent FR3123237A1, proposing a method for manufacturing a part, called a final part, comprising a portion of metallurgical material and a portion of at least one type of inorganic gemstone characterized by the following steps:

[0007] - use a powder of metallurgical material having a granulometry in less than 400 micrometers,

[0008] - reducing the size of the grains and / or crystallites of the metal material powder lurgical process so as to obtain aggregates with a characteristic size of less than 1000 micrometers, and an average crystallite size of less than 200 nanometers,

[0009] - using a powder of at least one type of inorganic gemstone having a particle size less than 500 micrometers,

[0010] - sintering using an SPS sintering process the mixture of said powders, such that the final part obtained has a predetermined shape and / or predetermined mechanical properties. This process relates in particular to the sintering of scraps or waste or powders or any elements resulting from the cutting of natural precious stones and provides for the mixing with a metallic element (iron, nickel, titanium or aluminum or a mixture thereof) acting as a dopant.

[0011] Patent EP3630704A1 is also known, describing a hydrothermal sintering process which is characterized by the fact that the material to be densified, for example an oxide, in powder / monolith form is in a liquid medium allowing, under the effect of pressure (up to 350 MPa) and temperature (up to 500°C), the dissolution-precipitation phenomena at the solid / liquid interfaces in “mild” conditions. It is thus possible to initiate the sintering phenomenon of metastable or amorphous phases at low temperature. The devices allow diameters to be obtained: 10 and 20 mm with various solvents: pure water, acid, base have been designed...

[0012] Patent US10730803B2 describes a method for preparing a sintered material, the method consisting of: - combine at least one inorganic compound in particulate form having a particle size of less than 50 qm with a solvent capable of partially solubilizing the inorganic compound to form a mixture; - applying pressure and heat to the mixture via an unsealed device exposed to atmospheric pressure to evaporate the solvent such that the evaporated solvent leaves the mixture and exits the unsealed device during the application of pressure and heat, the application of pressure and heat to the mixture densifies said at least one inorganic compound to a relative density greater than 80%, wherein the applied heat is at a temperature of at most 200°C above the boiling point of the solvent at atmospheric pressure and wherein the at least one inorganic compound and said at least one densified inorganic compound have the same composition. Disadvantages of the prior art

[0013] The solutions of the prior art are not entirely satisfactory because for some, the sintering temperature is high and for others the addition of dopants or solvents leads to denaturation of the ceramic obtained.

[0014] Furthermore, in patent EP3630704A1, the heating used (heating collar) for the double piston autoclave described is not suitable for allowing the passage of an electric current from one end to the other of the autoclave in SPS or FLASH type equipment. Solution provided by the invention

[0015] In order to overcome these drawbacks, the invention relates to a method for densifying materials or consolidating an assembly of materials comprising a single sintering step consisting of the simultaneous application inside a sintering chamber formed by a cavity hollowed out in a solid body, of a uniaxial force and a sintering temperature to said material or to said assembly placed in this sintering chamber, said force being applied by at least two pistons movable towards each other inside said sintering chamber., said pistons sliding in the bore hollowed out in said body and opening into said sintering chamber, the assembly formed by said sintering chamber, and said pistons being sealed characterized in that the sintering temperature is produced by the passage of an electric current in said body, said body being electrically and thermally conductive.

[0016] According to one variant, said material consists of a ceramic comprising an oxide.

[0017] The invention also relates to a method with a step of preparing a sample comprising a powder of said oxide and a hydrated precursor of said oxide without adding solvent to said sample.

[0018] Advantageously, said oxide is chosen from Al2O3, ZrO2 or MgAl2O4

[0019] According to one variant, said hydrated precursor is a hydrated phase of aluminum oxide. According to one variant, said hydrated precursor is any other hydrated phase for a material without aluminum.

[0020] According to one variant, said hydrated precursor consists of gibbsite A1(OH)3.

[0021] The invention also relates to a reactor for the densification of materials or consolidation of an assembly of materials comprising a solid body having a bore constituting the sintering chamber, as well as two pistons sliding in said bore and a means for heating said sintering chamber characterized in that said body is constituted by an electrically conductive material and in that said heating means is constituted by the Joule effect produced by the passage of an electric current in said body.

[0022] Advantageously, the head of said pistons is provided with a seal and in that the rod of said pistons is surrounded by a sheet of graphite or any other flexible sheet of electrically conductive material.

[0023] According to a variant, the reactor comprises a conductive or insulating pin arranged between said sealing joint and said sintering chamber.

[0024] Advantageously, said conductive or insulating pin has an annular groove forming a sintering chamber for collecting excess solvent from said material during the sintering phase.

[0025] Detailed description of a non-limiting example of embodiment

[0026] The present invention will be described in more detail with reference to non-limiting examples of embodiment specifying the above-mentioned advantages and considerations where:

[0027] [Fig.l] [Fig.l] represents a schematic view in longitudinal section of a reactor according to the invention

[0028] [Fig.2] [Fig.2] represents a schematic view in longitudinal section of the body of the reactor according to the invention

[0029] [Fig.3] [Fig.3] represents a schematic view in longitudinal section of a reactor according to an alternative embodiment of the invention

[0030] [Fig.4] [Fig.4] represents a schematic view in axisymmetric longitudinal section of a reactor according to the invention during SPS sintering (electric current density vector) of an electrically conductive sample (5) (case in the presence of solvent)

[0031] [Fig.5] [Fig.5] represents a schematic view in axisymmetric longitudinal section of a reactor according to the invention during SPS sintering ((electric current density vector) of an electrically insulating sample (5) (case in the absence of solvent) General principle of the invention

[0032] The principle of the invention is to combine SPS sintering technology with the hydrothermal process to apply this process to conventional SPS equipment, thanks to a new configuration of the high pressure reactor.

[0033] The originality of the process is based on the design of the HP reactor allowing it to withstand a pressure greater than 100 MPa (performance of a conventional graphite mold used in SPS equipment) replaced by a more mechanically efficient material while being electrically conductive because the body of the reactor generates heating by the Joule effect. The assembly of this HP reactor ensures sufficient sealing to ensure the presence of the hydro (solvo) thermal fluid (hydrothermal for an aqueous solvent or solvothermal for a non-aqueous solvent).

[0034] The reactor consists mainly of a solid body (3), crossed by a cylindrical bore defining two segments (1, 2) and a middle zone defining a sintering chamber in which the sample (5) is placed, subjected to the pressure of two pistons (10, 20) axially movable in the segments respectively (1, 2) by a hydraulic press for example, to apply to the two axial surfaces of the sample (5), arranged in the reaction chamber formed by the middle zone of the bore, both an antagonistic mechanical pressure as well as an electric current resulting from the potential difference applied to the two pistons (10, 20) forming electrodes transmitting the electric current to the sample (5).

[0035] The body (3) of which [Fig. 3] represents a schematic sectional view is a solid block made of an electrically and thermally conductive material, connected to two electrodes arranged at two axially opposite points, on either side of the middle zone of the bore, and at points as far apart as possible so that a potential gradient is established between the two electrodes, in the body (3), with a potential difference between the pistons (10 and 20).

[0036] This solid block (3) is crossed by a cylindrical bore, the upper section (1) of which ensures the guidance of one of the pistons (10) and a lower section (2) ensures the guidance of the other pistons (20). The intermediate segment between these two sections forms the reaction chamber in which the sample is treated.

[0037] Conventional SPS equipment allows the application and regulation of electric current for heating thanks to its electric generator; and also the application of force and its regulation thanks to its hydraulic press.

[0038] The HP reactor according to the invention is based on a hydro(solvo)thermal design in order to allow the passage of electric current from one electrode to the other electrode, arranged on either side of the conductive body. The passage of current between the movable pistons (10, 20) and the sample placed in the sintering chamber (3) is facilitated by the use of a flexible graphite cylinder, for example of the Papyex® brand (11) in order i. to facilitate the passage of current between the pistons (10, 20) and the sample placed in the sintering chamber, ii. (ii) to avoid electric arcs between the moving parts, in particular between the cylindrical surface of the pistons (10, 20) and the inner surface of the bore (1, 2) made in the solid body (3), and iii. (iii) displacements induced by chemical phenomena during the thermal process.

[0039] The seal is ensured by a polymer seal, by definition electrically insulating (15) reusable depending on the temperature experienced, without blocking the passage of the electric current and having good mechanical resistance to temperature, for example in Polybenzi-midazole - PBL. The innovative assembly of the seal and its associated piston ensures a hot seal up to more than IGPa thanks to the original design.

[0040] The body (3) of the HP reactor is made of an electrically conductive material, such as a metal alloy (steel, Nickel alloy, etc.) or a metal matrix composite (WC-Co cermet, etc.) or conductive ceramic (SiC, MoSi2, etc.) having high mechanical performance at temperature so as to reach at least IGPa and being able to be heated up to 1000°C depending on the type of material constituting the body (3) of the reactor.

[0041] Preferably, the examples given are obtained in the range 0.1 MPa - 1000 psi and for the temperature in the range 20°C - 500°C.

[0042] Description of an exemplary embodiment of the reactor according to the invention

[0043] Figures 1 and 2 represent an exemplary embodiment of an HP reactor according to the invention, the support pins of which have a circular groove or without a groove.

[0044] The equipment consists of a solid body (3) and comprises two pistons (10, 20) actuated axially in opposite directions by a press, one towards the other inside a sintering chamber for the application of a uniaxial force of the sample (5) to be treated. Each piston (10, 20) has a diameter reduction in order to position the graphite sheet (11) ensuring electrical continuity between the sintering chamber and the piston with a conical end intended to fit into the sealing element (15), for example a PBI seal which bears on the pin in contact with the sample (5) to be treated in the sintering chamber. These support pins can be perfectly cylindrical (13) or have a groove (14) determined by a circular groove to collect the excess fluid (24) in liquid form evacuated during the sintering step. Temperature regulation is done with the thermocouple positioned in its housing (9).

[0045] [Fig.3] illustrates the current flows in the body (3) when the sample (5) is made of a conductive material. [Fig.4] illustrates the current flows in the body (3) when the sample (5) is made of a weakly conductive material. The body (3) is supplied by two electrodes (26, 27) arranged on either side of the sintering chamber, on longitudinally opposite parts.

[0046] Method for sintering a ceramic sample (5) according to the invention

[0047] The method of sintering a sample (5) of oxide described below is advantageously implemented in a reactor in accordance with the invention described previously.

[0048] However, this method can also be implemented in a traditional reactor; its main characteristic concerns the fact that it does not provide for the addition of a solvent as is provided in the prior art but that the solvent is produced at the time of hydrothermal sintering by SPS effect by the reaction between the oxide and a hydrated precursor of an oxide delivering the water molecules at the origin of the solvent in the hydrothermal state. This method is therefore not limited to sintering in a reactor heated by Joule effect by a current passing through the body (3).

[0049] This process is particularly applicable to the recovery of powder from the grinding of materials used in the cut stone jewelry industry, which are currently not recovered. It concerns, for example, sapphire (A12O3) or oxides such as MgAl2O4 (spinel) and ZrO2. Hydrothermal sintering is used to obtain a ceramic at a lower temperature than conventional processes, based on the dissolution-precipitation phenomenon. This hydrothermal sintering process makes it possible to lower the sintering temperature, which is part of an eco-design approach.

[0050] The present invention is distinguished in particular from the hydrothermal sintering solutions of the prior art by the fact that the hydrothermal fluid does not come from a solvent but from a solid precursor; but can also be applied in a traditional manner by adding the solvent to the powder when loading the HP reactor.

[0051] For example, applied to sapphire powders, its precursor, gibbsite A1(OH)3, makes it possible not to introduce impurities from the solvent containing a mineralizer (NaOH, KOH, etc.) into the sapphire; which makes it possible, from the sapphire (A12O3) to be recovered, to propose a ceramic with no other element than Al, O, and H. The choice of gibbsite A1(OH)3 is motivated by the fact that the latter, in solid form, releases H20 molecules during the thermal decomposition of this aluminum hydrate, constituting the hydrothermal fluid activating hydrothermal sintering.

[0052] For example, conditions of 200°C for 1 hour lead to a very well sintered ceramic with smooth surfaces and capable of resisting a diametrical compressive stress at break of 600N (Brazilian test) on a diameter of 10mm.

[0053] This method according to the invention allows the adjustment of two pressures: • hydrothermal pressure, generated by the decomposition of the hydrated precursor or by the addition of solvent in the closed volume of the reactor and • the physical pressure generated by the application of the external force delivered by the press.

[0054] This method according to the invention makes it possible to adjust the hydrothermal pressure by means of the external force applied by the press.

[0055] This method according to the invention is activated by the passage of the electric current within the material by means of the fluid (electrical conductor), filling the volume of the reactor in addition to the temperature generated by the passage of the electric current in the HP reactor (heating element allowing the temperature to increase by the Joule effect).

Claims

Claims

1. Method for densifying materials or consolidating an assembly of materials comprising a single sintering step consisting of the simultaneous application inside a sintering chamber formed by a middle zone of a bore in a body (3), of a uniaxial force and a sintering temperature to said material or assembly placed in this sintering chamber, said force being applied by at least two pistons (10, 20) movable towards each other inside said bore, said pistons (10, 20) sliding in said bore (1; 2) in said body (3), the assembly formed by said bore (1, 2) and said pistons (10, 20), being sealed characterized in that the sintering temperature is produced by the passage of an electric current in said body (3), said body (3) being electrically and thermally conductive.

2. Method for densifying materials or consolidating an assembly of materials according to claim 1 characterized in that said material consists of a ceramic comprising an oxide.

3. Method for densifying materials or consolidating an assembly of materials according to the preceding claim, characterized in that it comprises the steps of preparing a sample (5) comprising a powder of said oxide and a hydrated precursor of said oxide without adding solvent to said sample (5).

4. Method for densifying materials or consolidating an assembly of materials according to claim 2 or 3, characterized in that said oxide is chosen from A12O3, ZrO2 or MgAl2O4

5. Method for densifying materials or consolidating an assembly of materials according to claim 3 or 4, characterized in that said hydrated precursor is a hydrated phase of aluminum oxide or any other hydrated phase for a material without aluminum.

6. Method for densifying materials or consolidating an assembly of materials according to the preceding claim, characterized in that said hydrated precursor consists of gibbsite A1(OH)3.

7. Reactor for the densification of materials or consolidation of an assembly of materials comprising a solid body (3) having a bore whose middle part forms a sintering chamber and whose two opposite segments (1, 2) receive two axially movable pistons (10, 20), the reactor further comprising a heating means of said sintering chamber characterized in that said body (3) is constituted by an electrically conductive material and in that said heating means is constituted by the Joule effect produced by the passage of an electric current in said body (3).

8. Reactor for the densification of materials or consolidation of an assembly of materials according to the preceding claim, characterized in that the head of said pistons (10, 20) is provided with a seal (15) and in that the rod of said pistons is surrounded by a sheet of graphite (11) (or any other flexible sheet of electrically conductive material).

9. Reactor for the densification of materials or consolidation of an assembly of materials according to the preceding claim, characterized in that it comprises a conductive or insulating pin (13) arranged between said seal (15) and said sintering chamber.

10. Reactor for the densification of materials or consolidation of an assembly of materials according to the preceding claim, characterized in that said conductive or insulating pin (14) has an annular groove forming a sintering chamber for collecting the excess solvent of said material during the sintering phase.

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