Method for producing a high-purity, dense sintered sic material

Abstract

There is a need for a scalable manufacturing process of SiC sintered materials having a relative density higher than 98%, preferably higher than 98.5%, or even higher than 99%, and having a SiC mass content higher than 99%, excluding free carbon.SOLUTION: The invention relates to a polycrystalline silicon carbide sintered material consisting of silicon carbide grains having a median equivalent diameter of between 1 and 10 micrometers, said material having a total porosity of less than 2% by volume of said material and a mass content of silicon carbide, excluding free carbon, of at least 99%, in which the mass ratio of the content of SiC having a beta (β) crystal form to the content of SiC having an alpha (α) crystal form is less than 2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to sintered materials based on high purity silicon carbide (SiC), and more particularly to methods for producing such materials. [Background technology]

[0002] Silicon carbide materials have long been known for their high hardness, chemical inertia, heat and mechanical resistance, and thermal conductivity, making them candidates of choice for applications such as cutting or machining tools, turbine parts or pump elements subject to high wear, pipe valves for transporting corrosive products, substrates and membranes for the filtration or decontamination of gases or liquids, heat exchangers and solar absorbers, coatings or materials for thermochemical processing of reactors, especially those for etching, or substrates intended for the electronics industry, temperature sensors or heating resistors, high-temperature or high-pressure sensors or sensors for very harsh environments, igniters or magnetic susceptors that are more resistant to oxidation than those made of graphite, and even certain applications such as mirrors or other optical devices.

[0003] However, sintering polycrystalline silicon carbide materials that are very dense (i.e., have a relative density greater than 99%) and highly pure (i.e., have a mass content of SiC greater than 98.5%, or even a mass content of SiC greater than 99.0%) remains a technical challenge.

[0004] It has long been known how to obtain dense ceramic bodies of silicon carbide without resorting to sintering additives that form liquid phases at very high temperatures (above 1500° C.) that are detrimental to the mechanical behavior.

[0005] For example, US Pat. No. 4,004,934 discloses a method for pressureless solid-phase sintering at a temperature of 1900 to 2100°C of a preform obtained by cold pressing a mixture containing a highly pure powder of SiC in the beta crystalline form, carbon in the form of a phenolic resin, with the content of this element being 0.1 to 1.0% by mass relative to the SiC, and a compound of boron, with the content of this element being 0.3 to 3.0% by mass relative to the SiC.

[0006] More recently, US Patent Application Publication No. 2006 / 0019816 proposes a manufacturing method starting from a slip comprising silicon carbide particles, a carbon source in the form of a water-soluble resin, at 2-10% by weight of the SiC, and a boron source, e.g., boron carbide, at 0.5-2% by weight of the SiC.

[0007] More recently, WO2019132667 proposes a method for producing a homogeneous mixture by co-milling 94% alpha SiC particles, 1% boron carbide particles, and 5% carbon source in an aqueous medium, which can result in a sintered body with a relative density of 96-98% after pulverization (atomization), casting, and sintering without load in argon at above 2100°C.

[0008] However, these solutions make it impossible to obtain a final material with a SiC content greater than 98.5%, or even greater than 99%, taking into account the boron content and the unavoidable impurities associated with the starting powder.

[0009] Ana Lara et al., in their paper "Densification of additive-free polycristalline β-SiC by spark-plasma sintering" published in Ceramic International 38 (2012) 45-53, show that very high purity and 98% relative density material can be obtained by SPS sintering at 2100°C without additives, starting from ultra-high purity β-type SiC powder, but its size is nanometer, with particles or crystallites having a median size of 10 nanometers. The use of such powders poses many handling problems, making the process difficult to scale up industrially. Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, there is a need for a scalable manufacturing process for SiC sintered materials having a relative density of greater than 98%, preferably greater than 98.5%, or even greater than 99%, and having a SiC mass content, excluding free carbon, greater than 99%. [Means for solving the problem]

[0011] The applicant's research, which will be described later, has demonstrated a combination that makes it possible to achieve this objective in terms of composition, mixture blending and sintering technology. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention more particularly relates to a first aspect of a method for producing a polycrystalline silicon carbide sintered material, comprising the steps of: (a) producing a mineral feedstock comprising, preferably consisting essentially of, by mass: - at least 95%, preferably at least 97%, of silicon carbide particles in the form of a powder, the silicon carbide particles having a median size of 0.1 to 5 micrometers, the silicon carbide particles having a SiC mass content of more than 95%, preferably more than 97%, and the powder in beta crystalline form representing more than 90%, preferably more than 95%, of the total mass of silicon carbide, and at least one solid-phase sintering additive, preferably in powder form, comprising an element selected from aluminium, boron, iron, titanium, chromium, magnesium, hafnium or zirconium, preferably an element selected from B, Ti, Hf or Zr, preferably an element selected from B or Zr, even more preferably B, preferably with a purity of more than 98% by weight, the contribution of said element being between 0.1 and 0.8%, preferably between 0.2 and 0.7%, of the total weight of said silicon carbide particles, - a carbon source having an elemental carbon content (C) of more than 99% by mass, between 0.5 and 3%, preferably in the form of uncrystalline or amorphous graphite or in the form of carbon powder, the median diameter of which is less than 1 micrometer; (b) forming the feedstock material into the form of a preform, preferably by casting; (c) solid state sintering the preform under a pressure of more than 60 MPa, preferably more than 75 MPa, or even more than 80 MPa, at a temperature of more than 1800°C and less than 2100°C in a nitrogen atmosphere, preferably in a dinitrogen atmosphere.

[0013] According to other optional and advantageous additional features of the above method, The mass content of free or residual carbon in the powder of silicon carbide particles is less than 3%, preferably less than 2%, preferably less than 1.5%. Preferably, free carbon is present in the silicon carbide powder only in the form of unavoidable impurities. the mass content of free or residual silica in the powder of silicon carbide particles is less than 2%, preferably less than 1.5%, preferably less than 1%. the mass content of free or residual silicon in the powder of silicon carbide particles is less than 0.5%, preferably less than 0.1%. Preferably, free silica is present only in the form of unavoidable impurities. the mass content of the element aluminum (Al), in metallic and non-metallic form, in the powder of silicon carbide particles is less than 0.2%. Preferably, aluminum is present only in the form of unavoidable impurities. The mass content of the powder of silicon carbide particles relative to the sum of the elements sodium (Na) + calcium (Ca) + potassium (K) + magnesium (Mg) is less than 0.2%, preferably with said elements being present only in the form of unavoidable impurities. The mass content of the powder of silicon carbide particles in the sum of the contents of aluminum (Al), alkali, alkaline earth and rare earth elements is less than 0.5%. The rare earth elements are Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Preferably, all these elements are present only in the form of unavoidable impurities. The element contained in the sintering additive is preferably boron. Preferably, the sintering additive is boron carbide powder. According to a particular embodiment, the element contained in the sintering additive is zirconium. Preferably, the sintering additive is zirconium carbide powder. According to one possible embodiment, the sintering additive is zirconium boride powder. The median diameter of the sintered powder is less than 2 micrometers, preferably less than 1 micrometer. - The specific surface area of ​​silicon carbide powder in beta crystalline form is 5 cm 2 / g and / or 30cm 2 / g. - The silicon carbide powder in beta crystalline form is bimodal, having two peaks, and even more preferably has a first peak having a height of 0.2 to 0.4 microns (micrometers), and a second peak having a height of 2 to 4 microns (micrometers).

[0014] Any molding technique known to those skilled in the art can be applied depending on the dimensions of the part to be made, provided that all precautions are taken to avoid contamination of the preform. Thus, casting in a plaster mold can be accommodated by using a graphite medium between the mold and the preform, or by using oil to avoid excessive contact and wear of the mold due to mixing, and ultimately contamination of the preform. These controlled precautions used by those skilled in the art can also be applied to the other steps of the method. Thus, the mold or matrix used to contain the preform during sintering will preferably be made of graphite.

[0015] Hot pressing, hot isostatic pressing or SPS (spark plasma sintering) techniques are particularly suitable. Preferably, pressure-assisted sintering is carried out by SPS, which is a sintering process in which induction heating is carried out by passing a direct current into a graphite matrix in which the preform is placed. The average heating rate is preferably greater than 10°C / min and less than 100°C / min. The plateau time at the maximum temperature is preferably greater than 10 minutes. This time can be relatively long depending on the type of preform and the load of the furnace.

[0016] The nitrogen used in the sintering atmosphere in step (c) is greater than 99.99% by volume pure, or even greater than 99.999% by volume pure.

[0017] According to one possible embodiment, the optional addition of carbon may be carried out in a mass ratio of 0.15 to 0.25 times the mass content of free silica in the silicon carbide powder in the feedstock, thereby forming silicon carbide by reaction and thus removing the free silica.

[0018] Preferably, the carbon addition is less than 3% by weight of elemental carbon (C) based on the weight of silicon carbide of the mineral feedstock.

[0019] According to another possible embodiment, silicon (preferably in the form of a metal powder, the silicon (Si) elemental content of which is greater than 99% by mass and the median diameter of which is preferably less than 1 micrometer) may optionally be added to the feedstock in a mass ratio of 1.5 to 2.5 times the mass content of free carbon in the starting beta crystalline silicon carbide powder, thereby forming silicon carbide by reaction and thus removing the free carbon.

[0020] Preferably, the silicon addition is less than 2% by weight elemental silicon (Si) based on the weight of silicon carbide of the mineral feedstock.

[0021] The present invention also relates to a polycrystalline material consisting of sintered silicon carbide grains producible by the method described above, the total porosity of which, by volume percentage of the material, is less than 2%, preferably less than 1.4%, preferably less than 1.2%, more preferably less than 1%, the mass content of silicon carbide (SiC) of which, excluding free carbon, is at least 99%, and the mass ratio of the content of SiC in the beta crystalline form (β) to the content of SiC in the alpha crystalline form (α) of the material is less than 2. The polycrystalline material consists of silicon carbide grains having a median equivalent diameter of 1 to 10 micrometers.

[0022] Other optional and advantageous additional features of the material are as follows: the mass content of oxygen (O) of said material is less than 0.5%, preferably less than 0.4% or even less than 0.3%. Preferably, oxygen is present in the material only in the form of an unavoidable impurity. The total elemental content of sodium (Na) + potassium (K) + calcium (Ca) is cumulatively less than 0.5% of the mass of the material. Preferably, sodium, potassium and calcium are present in the material only in the form of unavoidable impurities. the sum of the mass contents of the following elements is less than 0.5% of the mass of the material: aluminum (Al); alkalis; alkaline earth metals; rare earth metals, including at least one element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, preferably present in the material only in the form of unavoidable impurities. the mass content of boron (B) in said material is greater than 0.1% and / or less than 0.7%, preferably less than 0.6%, by mass of said material. According to one possible embodiment, the mass content of boron (B) is less than 0.5% by mass of said material. the mass content of element zirconium (Zr) in said material is greater than 0.1% and / or less than 0.7%, preferably less than 0.6%, by mass of said material. According to one possible embodiment, the mass content of zirconium is less than 0.5% by mass of said material. the elemental content of molybdenum (Mo) is less than 0.2% by mass of said material, preferably less than 0.1% by mass of said material; the elemental content of titanium (Ti) is less than 0.5% by mass of said material, preferably less than 0.2%, preferably less than 0.1% by mass of said material; The elemental mass content of nitrogen (N) in the material is between 0.05 and 0.5%, preferably greater than or equal to 0.1 and / or less than 0.3%. the elemental mass content of iron (Fe) is less than 0.5% of the mass of said material, preferably iron being present in the material only in the form of an unavoidable impurity; - the material contains less than 1% by mass of silicon in forms other than silicon carbide SiC, preferably only in the form of unavoidable impurities. - The material contains less than 2% by mass of carbon, a form other than silicon carbide (SiC). the mass content of free or residual carbon in said material is less than 1.5%, preferably less than 1.0%; Preferably, carbon, in a form other than silicon carbide SiC, is present in the material only in the form of an unavoidable impurity. the mass content of free or residual silica in said material is less than 1.5%, preferably less than 1.0%, preferably less than 0.5%; the mass content of free or residual silicon in said material is less than 0.5%, preferably less than 0.1%; The SiC, including free carbon, represents more than 97%, preferably more than 98%, by mass of said material. the mass ratio of the content of SiC in the beta crystalline form (β) to the content of SiC in the alpha crystalline form (α) of said material is less than 1.5, preferably less than 1, or even less than 0.3, or even less than 0.2, or even less than 0.1. the mass ratio of the content of SiC in the beta crystalline form (β) to the content of SiC in the alpha crystalline form (α) of said material is greater than 0.01, more preferably greater than 0.02; the material comprises more than 1% by weight of SiC in beta crystalline form, preferably more than 3% by weight of SiC in beta crystalline form, relative to the total weight of the crystallized phases in the material. SiC in the beta crystalline form (β) preferably represents less than 50% by mass of the crystalline phases of the material. the silicon carbide grains represent at least 98% by weight, preferably 99% by weight, of said material, the remainder consisting essentially of a residual grain boundary phase comprising the elements Si and C, preferably consisting essentially of the elements Si and C. In the material according to the invention, nitrogen can be present in the grains by insertion into the crystal lattice of SiC. - more than 90%, preferably more than 95%, by volume of the particles of said material, excluding its porosity, have an equivalent diameter of between 1 and 10 microns (micrometers), preferably between 1 and 8 microns (micrometers). More than 90% by volume, preferably more than 95% by volume, and even more preferably all, of the silicon carbide grains that are in alpha crystalline form have an equivalent diameter of less than 10 microns (micrometers).

[0023] According to one possible embodiment, the present invention relates to a polycrystalline silicon carbide sintered material consisting of silicon carbide grains with a median equivalent diameter of 1 to 10 microns (micrometers), said material having a mass content of silicon carbide (SiC) of at least 99%, excluding a total porosity of less than 2% by volume of said material and excluding free carbon, wherein the mass ratio of the content of SiC having beta crystalline morphology (β) to the content of SiC having alpha crystalline morphology (α) in said material is less than 2, and has the following elemental composition (by weight): - less than 0.5% silicon in forms other than SiC; less than 2.0% of carbon in forms other than SiC, preferably less than 1.5% of carbon in forms other than SiC, in particular 0.5 to 1.5% of carbon in forms other than SiC, and - 0.1 to 0.7% in total of at least one element selected from Al, B, Fe, Ti, Cr, Mg, Hf or Zr, preferably said element selected from B, Zr, Hf or Ti, even more preferably said element being B, Zr or Ti, even more preferably said element being B, - less than 0.5% oxygen (O), and - less than 0.5% in total of the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and - less than 0.5% of alkali elements, and - less than 0.5% alkaline earth elements, and - 0.05-1% nitrogen (N), - Other elements that form a complement to 100%. wherein the mass ratio of the content of SiC in the beta crystalline form (β) to the content of SiC in the alpha crystalline form (α) in said material is less than 2.

[0024] The present invention also relates to a device comprising at least one component made of a material as described above, said device being selected from: a turbine, a pump, a valve or a fluid line system, a heat exchanger; a solar heat absorber or a device for heat recovery or light reflection, a refractory coating for a furnace, a cooking surface, a crucible for melting metals, a wear protection part, a cutting tool, a brake pad or a disk, a radome, a coating or a support for thermochemical processes, e.g. etching, or a substrate for the deposition of active layers for the optical and / or electronics industry; a heating element or resistor; a temperature or pressure sensor; an igniter; a magnetic susceptor.

[0025] definition

[0026] The following indications and definitions are provided in connection with the foregoing description of the invention: - polycrystalline material is understood to mean a material having crystals of more than one or different crystal orientations; In sintered ceramic materials, the grains together constitute a substantial part of the mass of the material, and the grain boundary phase optionally consists of a ceramic and / or metallic phase, or the residual carbon advantageously accounts for less than 5% of the mass of the material. Unlike so-called liquid phase sintering, the firing process of the material according to the invention is carried out essentially in the solid phase, i.e., it is sintering in which the additives added to enable sintering or the level of impurities optionally present do not allow the formation of a liquid phase in an amount sufficient to allow the grains to rearrange and thus contact each other. Materials obtained by solid phase sintering are generally called "solid phase sintered bodies." - sintering additives (often simply called "additives") are understood herein to mean compounds conventionally known to facilitate and / or accelerate the kinetics of the sintering reaction. - silicon carbide (or SiC) is understood to mean the reaction product of a silicon source Si and a carbon source C when the silicon and carbon elements are mixed in stoichiometric proportions. This reaction product is silicon carbide, essentially in beta crystalline form, at temperatures below 1600°C and in a non-oxidizing atmosphere.

[0027] A powder of silicon carbide particles essentially of beta crystalline form is understood to mean a powder in which the 3C or cubic crystalline form represents more than 90% by weight, preferably more than 95% by weight, of the silicon carbide. The alpha crystalline form of silicon carbide is predominantly hexagonal or rhombohedral: 3H; 4H; 6H and 15R.

[0028] The term "except for free carbon" is understood to mean all components of the material other than free carbon.

[0029] Impurities are understood to mean unavoidable components that are unintentionally and necessarily introduced with the raw materials or that result from reactions between components. Impurities are not necessary components, but only tolerated components.

[0030] The elemental chemical content of the sintered material or the powder used in the mixture for the production of said material is measured according to techniques well known in the art. In particular, the levels of elements such as Al, B, Ti, Zr, Fe, Hf, Mo, rare earth metals, alkali metals, and alkaline earth metals can be measured by X-ray fluorescence, preferably by ICP ("inductively coupled plasma"), depending on the levels present, particularly if the levels are less than 0.5% or even less than 0.2%, in particular for products sintered in air at 750°C until weight loss, according to the ISO 21068-3:2008 standard. The mass content of free silicon, free silica, free carbon, and SiC is measured according to the ISO 21068-2:2008 standard. Their oxygen and nitrogen content is determined, in particular by LECO, according to the ISO 21068-3:2008 standard.

[0031] The polytype composition of SiC and the presence of other phases in sintered materials or powders used in the mixtures used in the manufacturing process of said materials are usually obtained by X-ray diffraction and Rietveld analysis. In particular, the respective percentages of alpha and beta SiC phases can be determined using a D8 Endeavor instrument made by BRUKER with the following configuration: - Acquisition: d5f80: 5° to 80° in 2θ, 0.01° step, 0.34 seconds / step, 46 minutes duration, - Front optic: Primary slit 0.3°, Soller slit 2.5°, - Sample holder: automatic cutter with rotation speed of 5 rpm / min, - Rear optic: Soller slit: 2.5°; Nickel filter 0.0125mm; PSD: 4°. 1D detector (current value).

[0032] The diffractograms may be qualitatively analyzed using the software EVA and the ICDD2016 database, and they may be quantitatively analyzed following Rietveld refinement using the HighScore Plus software.

[0033] The volume percentage of grains in the sintered material that are in the alpha or beta form, and their diameters, can be determined by analyzing images obtained from electron backscatter diffraction (EBSD) observations. The equipment may consist, for example, of a scanning electron microscope (SEM) equipped with an EBSD detector and a spectrometer equipped with energy dispersive X-ray spectroscopy (EDX). The EBSD and EDX detectors are controlled by the software ESPRIT (version 2.1). Images with high crystallographic contrast and / or high density contrast can be collected using available software.

[0034] The equivalent diameter of a grain corresponds to the diameter of a disk of the same surface area as that of said grain as observed along a cutting plane of the material. By using various cross sections of the material along at least two perpendicular planes, one can have a very good representation of the volume distribution of the various equivalent diameters of the grains, from which the median equivalent diameter (or D) of said grains can be calculated by volume. 50 In this application, the volume percent of sintered grains that make up a material is expressed relative to the volume of the material excluding its porosity.

[0035] The median equivalent diameter of the grains corresponds to a diameter that divides the grains into first and second equal populations, each of which has only grains with equivalent diameters greater than the median diameter or only grains with equivalent diameters less than the median diameter.

[0036] Similarly to above, the volume of any grain boundary phases present may be calculated.

[0037] The total porosity (or total pore volume) of a material according to the invention corresponds to the sum of the closed and open pore volumes divided by the volume of the material, and is calculated according to the ratio, expressed as a percentage, of the bulk density, measured according to ISO 18754, to the true density, measured according to ISO 5018.

[0038] The median particle diameter (or median "size") of the particles constituting a powder can be obtained by characterizing the particle size distribution, in particular by means of a laser particle size analyzer. Characterization of the particle size distribution is conventionally carried out using a laser particle size analyzer according to the ISO 13320-1 standard. The laser particle size analyzer can be, for example, a Partica LA-950 from HORIBA. For the purposes of this specification, unless otherwise stated, the median diameter of particles refers to the diameter of the following particles, respectively, at which 50% by mass of the population are found. The "median diameter" or "median size" of a collection of particles, in particular of a collection of powders, is the diameter of the following particles, at which 50% by mass of the population are found: 50Percentiles are referred to as sizes that divide the particles into first and second populations of equal volume, each of which has only grains with an equivalent diameter greater than the median size or only grains with an equivalent diameter less than the median size, respectively.

[0039] A powder of particles of silicon carbide in the beta crystalline form is understood to mean a powder in which the 3C or cubic crystalline form represents more than 95% by mass of the silicon carbide. The alpha crystalline forms of SiC are predominantly the hexagonal or rhombohedral phase; 3H; 4H; 6H and 15R.

[0040] The specific surface area is measured by the BET (Brunauer Emmet Teller) method, which is described in, for example, Journal of American Chemical Society 60 (1938), pp. 309-316.

[0041] Unless otherwise specified, all percentages herein are by weight. [Example]

[0042] Illustrative Embodiments

[0043] Below we give non-limiting examples that make it possible to produce materials according to the invention, but which of course do not limit the methods that make it possible to obtain such materials and the methods according to the invention, as well as comparative examples that illustrate the advantages of the invention.

[0044] In all the following examples, ceramic bodies in the form of cylinders with a diameter of 30 mm and a thickness of 10 mm were first produced by casting a slip into a plaster mold from the following ingredients according to the various formulations reported in Table 1 below: (1) A powder of silicon carbide particles essentially of beta crystalline morphology, which, according to a non-cumulative particle size distribution measured by a laser particle size analyzer, is bimodal by number, having a first peak maximizing at 0.3 micrometers and a second peak substantially twice as high as the first peak maximizing at 3 micrometers. The median diameter of the bimodal powder is 1.5 micrometers. The SiC powder has the following elemental mass levels: Sc+Y+La+Ce+Pr+Nd+Pm+Sm+Eu+Gd+Tb+Dy+Ho+Er+Tm+Yb+Lu<0.5%; Nitrogen (N) <0.2%; Na+K+Ca+Mg<0.2%; Aluminum (Al) <0.1%; Iron (Fe)<0.05%; Titanium (Ti)<0.05%; Molybdenum (Mo)<0.05%; Its carbon, silica, and free silicon contents are less than 2.0%, less than 1.0%, and less than 0.1%, respectively. The mass content of the beta-SiC phase is greater than 95%. (2) Silicon carbide powder that is essentially in alpha crystalline form, having an alpha SiC content of greater than 95% by weight, with carbon, silica, and free silicon contents of less than 0.2%, less than 1.5%, and less than 0.1%, respectively. (3) Provided by Timcal in grade C65, 62m 2 / g BET specific surface area. (4) Boron carbide BC powder, supplied by H.C. Starck in grade HD-15, having a median diameter of 0.8 micrometers. (5) Aluminum nitride powder, offered in grades by Nanografi, with a median diameter of 0.06 micrometers.

[0045] The pellets thus produced are dried in air at 50° C. The pellets of Examples 1 and 2 (comparative examples) are sintered in a furnace without pressure at a temperature of 2150° C. for 2 hours in argon and N2, respectively. The pellets of Examples 3 and 4 (according to the invention) and Example 5 (comparative example) are loaded into an SPS sintering apparatus at 2000° C. in a nitrogen atmosphere under a load of 85 MPa (megapascals).

[0046] Unlike Examples 4 and 5, the B4C powder was replaced with aluminum nitride powder and sintering was carried out in a vacuum. Unlike Example 1, in Example 7 the starting powder was essentially beta and sintering was carried out in a vacuum and under pressure under the same conditions as Example 6.

[0047] The total porosity of the part obtained after sintering is calculated by taking the difference between 100 and the ratio, expressed as a percentage, of the bulk density measured according to ISO 18754 to the true density measured according to ISO 5018.

[0048] The free silica content (SiO2) is measured by HF attack. The free carbon, oxygen and nitrogen contents are measured by the LECO method. The levels of other elements are measured by X-ray fluorescence and ICP.

[0049] The free silicon content is measured by controlling with aqua regia and then by titration. The percentage of SiC in beta form and the ratio of SiC in crystalline form β / α are determined by X-ray diffraction analysis according to the method described above.

[0050] The volume percentage of grains in the sintered material that are in the alpha or beta form and their diameters were determined by analysis of images obtained from EBSD observations.

[0051] The equipment includes a Bruker e-FlashHR+ EBSD detector with an FSE / BSE Argus imaging system, and a 10mm 2The scanning electron microscope (SEM) consisted of a Bruker XFlash 4010 EDX detector with an active surface area of ​​10.5 mm. The EBSD detector was mounted on one of the rear ports of an FEI Nova NanoSEM 230 scanning electron microscope equipped with a field emission gun at a tilt angle equal to 10.6° relative to the horizontal to increase both the EBSD and EDX signals. Under these conditions, the optimal working distance (WD) (i.e., the distance between the SEM pole piece and the analyzed area of ​​the sample) was approximately 13 mm. The EBSD and EDS detectors were controlled by the software ESPRIT (version 2.1). FSE images (with high crystallographic contrast) and / or BSE images (with high density contrast) were collected using an Argus system by positioning the EBSD camera at a distance DD (sample-detector distance) of 23 mm to ensure that the images were relatively insensitive to the sample topography. EBSD measurements were performed in point scan and / or mapping modes. For this purpose, the EBSD camera was positioned at a distance DD of 17 mm to increase the collected signal.

[0052] The equivalent diameter of a grain corresponds to the diameter of a disk of the same surface area as that of said grain, as observed along a plane of cut of the material. By observing various cross sections of the material along at least two perpendicular planes, the distribution of the various equivalent diameters of the grains in the volume of the material could be determined, from which the median equivalent diameter of said grains by volume could be inferred.

[0053] The properties and characteristics obtained according to Examples 1 to 7 are shown in Table 1 below.

[0054] [Table 1]

[0055] ND = Not detectable NM = Not measured

[0056] The examples according to the invention show that high-purity, very dense crystallized silicon carbide materials can be obtained by a very special method, which involves mixing essentially beta-form silicon carbide SiC in the presence of carbon with moderate addition of sintering additives, and sintering under pressure in a pure nitrogen atmosphere. Examples 6 and 7 (comparative examples) show that vacuum sintering, whether using sintering additives that provide nitrogen (Example 6) or not (Example 7), does not allow for the production of dense SiC materials, i.e., with a porosity of less than 2%, or even less than 1%, and with a median equivalent grain diameter of 1 to 10 micrometers, unlike the method according to the invention.

Claims

1. 1. A polycrystalline silicon carbide sintered material consisting of silicon carbide grains having a median equivalent diameter of 1 to 10 micrometers, said material having a mass content of silicon carbide (SiC) of at least 99%, excluding free carbon and a total porosity of less than 2% by volume of said material, wherein the mass ratio of the content of silicon carbide having a beta (β) crystal morphology to the content of silicon carbide having an alpha (α) crystal morphology in said material is less than 2, and said material has the following elemental composition, by mass: - less than 0.5% of silicon in forms other than silicon carbide; less than 2.0% carbon in forms other than silicon carbide, preferably less than 1.5% carbon in forms other than silicon carbide, and - 0.1 to 0.7% in total of at least one element selected from Al, B, Fe, Ti, Cr, Mg, Hf, Zr, preferably Zr, Ti, Hf, B; less than 0.5% oxygen (O), and less than 0.5% in total of the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and - less than 0.5% of alkali elements, and - less than 0.5% alkaline earths, and - 0.05 to 1% nitrogen (N), - Other elements that form a complement to 100%.

2. 10. The material of claim 1, wherein the material comprises greater than 1% SiC in the beta crystalline form, based on the total mass of crystalline phases in the material.

3. 3. The material of claim 1 or claim 2, wherein more than 90% of the grains, by volume excluding porosity, of the material have an equivalent diameter of 1 to 10 micrometers.

4. The material according to any one of claims 1 to 3, wherein the element mass content of nitrogen (N) in the material is 0.05 to 0.5% by mass.

5. The material according to any one of claims 1 to 4, wherein the mass content of boron (B) is more than 0.1 mass% and less than 0.7 mass% of the material.

6. 6. The material according to claim 1, wherein the mass ratio of the content of SiC in the beta crystalline form (β) to the content of SiC in the alpha crystalline form (α) in the material is less than 1.

7. 7. The material according to any one of claims 1 to 6, wherein silicon carbide grains represent at least 98%, preferably 99%, by weight of the material, the remainder consisting essentially of a residual grain boundary phase comprising, preferably consisting essentially of, the elements Si and C.

8. 8. The material of any one of claims 1 to 7, wherein more than 90% by volume of the silicon carbide grains in the alpha crystalline form have an equivalent diameter of less than 10 micrometers.

9. A method for producing a polycrystalline silicon carbide sintered material according to any one of claims 1 to 8, comprising the steps of: (a) generating a mineral feedstock comprising, by mass: - at least 95%, preferably at least 97%, of silicon carbide particles in the form of a powder, the median size of which is between 0.1 and 5 micrometers, the silicon carbide particles having a SiC mass content of more than 95%, preferably more than 97%, and the beta crystalline form representing more than 90%, preferably more than 95%, of the total mass of silicon carbide; at least one solid-phase sintering additive, preferably in powder form, comprising an element chosen from aluminium, boron, iron, titanium, chromium, magnesium, hafnium or zirconium, preferably with a purity of more than 98% by weight, said element contributing between 0.1 and 0.8% of the total weight of the silicon carbide particles; - 0.5 to 3% of a carbon source having an elemental carbon content (C) of more than 99% by mass, preferably in the form of uncrystalline or amorphous graphite or in the form of carbon powder, the median diameter of which is less than 1 micrometer, (b) forming the feedstock material into the form of a preform, preferably by casting; (c) solid state sintering the preform under a pressure greater than 60 MPa at a temperature greater than 1800°C and less than 2100°C in a nitrogen atmosphere, preferably a dinitrogen atmosphere.

10. 10. The method of claim 9, wherein the mass content of free carbon in the powder of silicon carbide particles is less than 2%.

11. 11. The method according to claim 9 or claim 10, wherein the mass content of free silica in the powder of silicon carbide particles is less than 1%.

12. 12. The method according to any one of claims 9 to 11, wherein the mass content of free silicon in the powder of silicon carbide particles is less than 0.5%.

13. 13. The method according to any one of claims 9 to 12, wherein the mass content of the powder of silicon carbide particles in the total elemental content of aluminum (Al), alkali, alkaline earth, and rare earth metals including at least one element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu is less than 0.5%.

14. The method according to any one of claims 9 to 13, wherein the element contained in the sintering additive is boron.

15. The method according to any one of claims 9 to 14, wherein the step of solid-state sintering the preform is carried out by SPS ("Spark Plasma Sintering").

16. 10. An apparatus comprising a material according to any one of claims 1 to 9, said apparatus being selected from the following: a turbine, a pump, a valve or a fluid line system, a heat exchanger; a solar heat absorber or an apparatus for heat recovery or light reflection, a refractory coating for a furnace, a cooking surface, a crucible for melting metals, a wear protection part, a cutting tool, a brake pad or a disc, a radome, a coating or a support for thermochemical processes or a substrate for the deposition of active layers for the optical and / or electronic industry; a heating element or resistor; a temperature or pressure sensor; an igniter; a magnetic susceptor.

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