Method for producing high purity, dense sintered sic material

Abstract

The invention relates to a polycrystalline silicon carbide sintered material consisting of silicon carbide grains having a median equivalent diameter of 1 to 10 microns, the total porosity of the material being less than 2% of the volume of the material and the mass content of silicon carbide, excluding free carbon, being at least 99%, wherein in the material the mass ratio of the content of SiC having the beta type crystal form to the content of SiC having the alpha type crystal form is less than 2. A method for producing this material is also disclosed.

Description

[0001] This invention relates to sintered materials based on high-purity silicon carbide (SiC), and more particularly to methods for manufacturing such materials.

[0002] Silicon carbide materials have long been known for their high hardness, chemical inertness, heat resistance, mechanical resistance, and thermal conductivity. This makes them ideal for applications such as cutting or machining tools; turbine components or pump elements subjected to high wear; pipeline valves for conveying corrosive products; supports and membranes intended for filtering or removing contaminated or liquid substances; supports and membranes for filtering or removing gases or liquids; heat exchangers and solar absorbers; coatings or materials for thermochemical treatments in reactors (especially for etching); or substrates intended for use in the electronics industry; temperature sensors or heating resistors; high-temperature or pressure sensors or sensors for very harsh environments; igniters or magnetoresistors that are more oxidation-resistant than those made of graphite; and even certain special applications, such as mirrors or other optical devices.

[0003] However, sintering polycrystalline silicon carbide materials with very high density (i.e., relative density greater than 99%) and high purity (i.e., SiC mass content greater than 98.5%, or even SiC greater than 99.0%) remains a technical challenge.

[0004] Methods for obtaining dense silicon carbide ceramic bodies are well known and do not require sintering additives that form a liquid phase at very high temperatures (>1500°C) that is detrimental to mechanical properties.

[0005] For example, US4004934 discloses a method for pressureless solid-state sintering of a preform at a temperature of 1900 to 2100 °C, the preform being obtained by cold pressing a mixture comprising very pure β-crystalline SiC powder and carbon in the form of phenolic resin and boron compounds, wherein the mass of carbon is 0.1 to 1.0% relative to SiC and the mass of boron is 0.3 to 3.0% relative to SiC.

[0006] Recently, US2006 / 0019816 disclosed a method for manufacturing a slurry comprising silicon carbide particles, a carbon source in the form of a water-soluble resin comprising 2 to 10% by mass of SiC, and a boron source such as boron carbide comprising 0.5% to 2% by mass of SiC.

[0007] Recently, WO2019132667A1 proposed a method for producing a homogeneous mixture by co-milling in an aqueous medium containing 94% αSiC particles, 1% boron carbide particles, and 5% carbon source. This allows for the production of sintered bodies with a relative density of 96% to 98% after spraying, casting, and unloaded sintering in argon at temperatures above 2100°C.

[0008] However, given the boron content and the unavoidable impurities associated with the starting powder, these solutions cannot yield a final material with a SiC content greater than 98.5% or even greater than 99%.

[0009] The paper "Densification of additive-Free polycristalline β-SiC by spark-plasmasintering" published by Ana Lara et al. in Ceramic International 38 (2012) 45-53 shows that a material with very high purity and a relative density of 98% can be obtained by SPS sintering at 2100 °C from ultrapure β-type SiC powder without any additives, but its size is in the nanoscale, with a median particle or crystallite size of 10 nm. The use of this powder brings many processing problems and makes this method difficult to scale up industrially.

[0010] Therefore, a method for the large-scale manufacture of sintered SiC materials is needed, with a relative density greater than 98%, preferably greater than 98.5%, or even greater than 99%, and a SiC mass content (excluding free carbon) greater than 99%. Summary of the Invention:

[0011] As described below, the applicant company’s work has demonstrated that the combination of composition, mixture formulation and sintering technology makes this goal possible.

[0012] More specifically, in a first aspect, the present invention relates to a method for manufacturing polycrystalline sintered silicon carbide materials, the method comprising the following steps:

[0013] a) Preparing mineral raw materials, which, by mass, contain the following components, preferably substantially composed of the following components:

[0014] - At least 95%, preferably at least 97%, of silicon carbide particles in powder form, with a mean size of 0.1 to 5 micrometers, and a SiC mass content greater than 95%, preferably greater than 97%, wherein β-crystalline powder accounts for greater than 90%, preferably greater than 95%, of the total mass of silicon carbide.

[0015] - At least one solid-state sintering additive, preferably in powder form, comprising an element selected from aluminum, boron, iron, titanium, chromium, magnesium, hafnium, or zirconium, preferably selected from B, Ti, Hf, or Zr, preferably selected from B or Zr, even more preferably B, preferably with a purity greater than 98% by mass, wherein the amount of said element is such that its contribution accounts for 0.1% to 0.8% of the total mass of the silicon carbide particles, preferably 0.2% to 0.7%.

[0016] A carbon source comprising 0.5% to 3% of the total carbon content (C) of 99% by mass, preferably in the form of amorphous or non-crystalline graphite or carbon powder, wherein the median diameter is less than 1 micrometer.

[0017] b) Forming the raw material into a preform, preferably by casting.

[0018] c) The preform is solid-state sintered in a nitrogen atmosphere, preferably in a nitrogen atmosphere, at a pressure greater than 60 MPa, preferably greater than 75 MPa, or even greater than 80 MPa, and at a temperature greater than 1800°C and less than 2100°C.

[0019] According to other optional and advantageous additional features of the method:

[0020] The mass content of free carbon or residual carbon in the silicon carbide powder is less than 3%, preferably less than 2%, and more preferably less than 1.5%. Preferably, free carbon exists only in the silicon carbide powder as an unavoidable impurity.

[0021] - The mass content of free or residual silica in silicon carbide particles is less than 2%, preferably less than 1.5%, and more preferably less than 1%.

[0022] - The mass content of free silicon or residual silicon in silicon carbide particles is less than 0.5%, preferably less than 0.1%.

[0023] - Preferably, free silica exists only as an unavoidable impurity.

[0024] - The mass content of aluminum (Al) in both metallic and non-metallic forms in the silicon carbide powder is less than 0.2%. Preferably, aluminum exists only as an unavoidable impurity.

[0025] The silicon carbide powder contains less than 0.2% by mass in terms of the sum of the elements sodium (Na), calcium (Ca), potassium (K), and magnesium (Mg). Preferably, these elements are present only as unavoidable impurities.

[0026] The total mass content of aluminum (Al), alkali metals, alkaline earth metals, and rare earth metals in the silicon carbide powder 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 exist only as unavoidable impurities.

[0027] - The element contained in the sintering aid is preferably boron. Preferably, the sintering additive is boron carbide powder.

[0028] - According to a specific embodiment, the element contained in the sintering additive is zirconium. Preferably, the sintering additive is zirconium carbide powder. According to one possible approach, the sintering additive is zirconium boride powder.

[0029] - The median diameter of the sintered powder is less than 2 micrometers, preferably less than 1 micrometer.

[0030] -β-crystalline silicon carbide powder has a specific surface area greater than 5 cm². 2 / g and / or less than 30cm 2 / g.

[0031] -β-crystalline silicon carbide powder is bimodal and has two peaks, even more preferably a first peak with a high point of 0.2 to 0.4 micrometers and a second peak with a high point of 2 to 4 micrometers.

[0032] Provided all precautions are taken to avoid contamination of the preform, any molding technique known to those skilled in the art can be applied, depending on the size of the part to be manufactured. Therefore, casting in the plaster mold can be regulated by using a graphite medium or oil between the mold and the preform, thereby avoiding excessive contact and wear of the mold due to mixing, and ultimately, contamination of the preform. These controlled precautions, available to those skilled in the art, also apply to other steps of the method. Therefore, during sintering, the mold or substrate used to house the preform is preferably made of graphite.

[0033] Hot pressing, hot isostatic pressing, or SPS (spark plasma sintering) technology is particularly suitable. Preferably, pressure-assisted sintering is performed by SPS, a sintering process in which induction heating is achieved by flowing a direct current into a graphite matrix in which the preform is placed. The average heating rate is preferably greater than 10 and less than 100°C / min. The plateau time at the highest temperature is preferably greater than 10 minutes. This time may be longer, depending on the specifications of the preform and the furnace load.

[0034] The nitrogen used in step c) for the sintering atmosphere has a purity greater than 99.99% by volume, or even greater than 99.999% by volume.

[0035] According to one possible implementation, carbon can be optionally added at a mass ratio of 0.15 to 0.25 times the mass content of free silica in the silicon carbide powder in the raw material, so as to form silicon carbide through reaction, thereby eliminating such free silica.

[0036] Preferably, the added carbon is less than 3% elemental carbon (C) based on the mass of silicon carbide relative to the mineral raw material.

[0037] According to another possible implementation, silicon (preferably in the form of a metal powder having an elemental content of silicon (Si) greater than 99% by mass and a mean diameter preferably less than 1 micrometer) may optionally be added to the raw material, wherein the mass ratio is 1.5 to 2.5 times the mass content of free carbon in the initial β-crystal form of the silicon carbide powder, so as to remove such free carbon by reacting to form silicon carbide.

[0038] Preferably, the added silicon is less than 2% by mass of elemental silicon (Si) relative to the mass of silicon carbide in the mineral raw material.

[0039] The present invention also relates to a polycrystalline material composed of sintered silicon carbide grains that can be manufactured by the above method, having a total porosity of less than 2%, preferably less than 1.4%, more preferably less than 1.2%, and more preferably less than 1%, wherein the material has a silicon carbide (SiC) content of at least 99% by volume percentage, excluding free carbon, and the mass ratio of β-type (β) SiC content to α-type (α) SiC content in the material is less than 2. The polycrystalline material is composed of silicon carbide grains with a median equivalent diameter of 1 to 10 micrometers.

[0040] According to other optional and advantageous additional features of the material:

[0041] - The oxygen (O) content of the material is less than 0.5% by mass, preferably less than 0.4%, or even less than 0.3%. Preferably, oxygen exists in the material only as an unavoidable impurity.

[0042] The total elemental content of sodium (Na), potassium (K), and calcium (Ca) is less than 0.5% of the mass of the material. Preferably, sodium, potassium, and calcium exist in the material only as unavoidable impurities.

[0043] The sum of the mass contents of aluminum (Al), alkali metals, alkaline earth metals, and rare earth metals containing 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% of the mass of the material. Preferably, the element exists in the material only as an unavoidable impurity.

[0044] The boron (B) content in the material is greater than 0.1% and / or less than 0.7% by mass of the material, preferably less than 0.6%. In one possible manner, the boron (B) content is less than 0.5% by mass of the material.

[0045] The zirconium (Zr) content in the material is greater than 0.1% and / or less than 0.7% by mass of the material, preferably less than 0.6%. In one possible manner, the zirconium content is less than 0.5% by mass of the material.

[0046] The molybdenum (Mo) content is less than 0.2% of the material mass, preferably less than 0.1% of the material mass.

[0047] - The titanium (Ti) content is less than 0.5% of the material mass, preferably less than 0.2% of the material mass, and more preferably less than 0.1% of the material mass.

[0048] The nitrogen (N) content in the material is 0.05% to 0.5% by mass, preferably greater than or equal to 0.1% and / or less than 0.3%.

[0049] The iron (Fe) element content is less than 0.5% of the mass of the material. Preferably, iron exists in the material only as an unavoidable impurity.

[0050] - Silicon in forms other than silicon carbide (SiC) constitutes less than 1% of the mass of the material. Preferably, silicon in forms other than silicon carbide (SiC) exists in the material only as unavoidable impurities.

[0051] - Carbon in forms other than silicon carbide (SiC) accounts for less than 2% of the mass of the material.

[0052] - The mass content of free carbon or residual carbon in the material is less than 1.5%, preferably less than 1.0%.

[0053] - Preferably, carbon in forms other than silicon carbide (SiC) exists in the material only as unavoidable impurities.

[0054] The mass content of free or residual silica in the material is less than 1.5%, preferably less than 1.0%, and more preferably less than 0.5%.

[0055] The mass content of free silicon or residual silicon in the material is less than 0.5%, preferably less than 0.1%.

[0056] -SiC accounts for more than 97% of the mass of the material including free carbon, preferably more than 98%.

[0057] - The mass ratio of the SiC content of the β-crystal (β) form to the SiC content of the α-crystal (α) form of the 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.

[0058] - The mass ratio of the SiC content of the β-crystal (β) SiC to the SiC content of the α-crystal (α) SiC in the material is greater than 0.01, more preferably greater than 0.02.

[0059] - The material contains more than 1% by mass of β-type SiC, preferably more than 3% by mass, relative to the total mass of the crystalline phases in the material.

[0060] -β-crystal (β) SiC preferably accounts for less than 50% of the crystal phase mass of the material.

[0061] - Silicon carbide grains account for at least 98%, preferably 99%, of the material mass, with the remainder consisting of residual intergranular phases containing elements Si and C, preferably substantially composed of elements Si and C.

[0062] - In the material according to the invention, nitrogen can exist in the grains by intercalation into the SiC lattice.

[0063] - The constituent grains of the material, excluding its porosity, comprise more than 90%, preferably more than 95%, of an equivalent diameter of 1 to 10 micrometers, preferably 1 to 8 micrometers, based on the volume of the material.

[0064] - By volume, more than 90%, preferably more than 95%, and even more preferably, all α-silicon carbide grains have an equivalent diameter of less than 10 micrometers.

[0065] According to one possible embodiment, the present invention relates to a polycrystalline silicon carbide sintered material composed of silicon carbide grains with a median equivalent diameter of 1 to 10 micrometers, wherein the total porosity of the material is less than 2% of the material volume, and the mass content of silicon carbide (SiC) excluding free carbon is at least 99%, wherein the mass ratio of the SiC content having β-type crystal form (β) to the SiC content having α-type crystal form (α) in the material is less than 2, and has the following elemental composition by weight:

[0066] - Less than 0.5% of silicon in forms other than SiC,

[0067] - Less than 2.0% of carbon other than SiC, preferably less than 1.5% of carbon other than SiC, particularly 0.5% to 1.5% of carbon other than SiC, and

[0068] - At least one element selected from Al, B, Fe, Ti, Cr, Mg, Hf, or Zr, totaling 0.1% to 0.7%, preferably selected from B, Zr, Hf, or Ti, even more preferably B, Zr, or Ti, and even more preferably B.

[0069] - Less than 0.5% oxygen (O), and

[0070] - Elements totaling less than 0.5% include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0071] - Less than 0.5% of alkali metal elements, and

[0072] - Less than 0.5% of alkaline earth metals, and

[0073] -0.05% to 1% nitrogen (N),

[0074] - Replenish other elements to 100%,

[0075] Furthermore, the mass ratio of the SiC content of the β-crystal form (β) to the SiC content of the α-crystal form (α) in the material is less than 2.

[0076] The present invention also relates to an apparatus comprising at least one component made of the aforementioned materials, the apparatus being selected from: turbines, pumps, valves or fluid pipeline systems, heat exchangers; solar absorbers or devices for recovering heat or reflecting light, furnace refractory coatings, cooking surfaces, crucibles for melting metal, wear protection components, cutting tools, brake pads or brake discs, radomes, coatings or supports for thermochemical treatments such as etching, or substrates for active layer deposition in the optical and / or electronics industries; heating elements or resistors; temperature or pressure sensors; igniters; and magnetoresistors.

[0077] definition:

[0078] The following descriptions and definitions are given in conjunction with the foregoing description of the present invention:

[0079] - Polycrystalline materials should be understood as materials with multiple crystal orientations or crystals with different crystal orientations.

[0080] - In sintered ceramic materials, the grains together constitute the majority of the material's mass, with the intergranular phase, optionally composed of ceramic and / or metallic phases or residual carbon, advantageously comprising less than 5% of the material's mass. Unlike so-called liquid-phase sintering, the method of sintering the material according to the invention is carried out essentially in the solid phase; that is, it is a sintering in which the level of sintering-allowing additives or optionally present impurities is unlikely to result in the formation of a liquid phase sufficient to allow grain rearrangement and thus contact between them. Materials obtained by solid-phase sintering are generally referred to as "solid-phase sintered."

[0081] - Sintering additives, usually referred to simply as "additives", should be understood in this specification as compounds generally known to be used to achieve and / or accelerate the kinetics of sintering reactions.

[0082] Silicon carbide (or SiC) should be understood as the reaction product between a silicon source and a carbon source, which is a mixture of elemental silicon (Si) and elemental carbon (C) in a stoichiometric ratio. The product of this reaction at temperatures below 1600°C and in a non-oxidizing atmosphere is essentially β-crystalline silicon carbide.

[0083] The term "silicon carbide powder with β-crystal structure" should be understood to refer to powder in which the 3C or cubic crystal structure accounts for more than 90% of the silicon carbide mass, preferably more than 95%. The α-crystal structure of silicon carbide is mainly hexagonal or rhombohedral phase; 3H; 4H; 6H and 15R.

[0084] The term "excluding free carbon" is understood to refer to all components of the material other than free carbon.

[0085] Impurities should be understood as unavoidable components, which are introduced unintentionally or necessarily with the raw materials or produced by reactions between components. Impurities are not necessary components, but only permissible ones.

[0086] The elemental chemical content of powders in sintered materials or mixtures used in methods for manufacturing said materials is measured according to techniques known in the art. Specifically, 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. If the levels are less than 0.5%, or even less than 0.2%, it is particularly important to measure by ICP, especially according to ISO 21068-3:2008, for calcined products at 750°C in air until the weight is absorbed. The mass content of free silicon, free silica, free carbon, and SiC is measured according to standard ISO 21068-2:2008. These oxygen and nitrogen content are determined by LECO, specifically according to ISO 21068-3:2008.

[0087] The polymorphic composition of SiC and the presence of other phases in the powder of the sintered material or mixture used in the method of manufacturing said material are typically determined by X-ray diffraction and Rietveld analysis. In particular, the respective percentages of α and β SiC phases can be determined using a BRUKER-manufactured D8 Endeavor apparatus with the following configuration:

[0088] Acquisition: d5f80: 2θ range from 5° to 80°, 0.01° step size, 0.34 seconds / step, duration 46 minutes.

[0089] - Front optics: Primary slit 0.3°; Soler slit 2.5°

[0090] -Sample holder: Automatic cutter rotating at 5 rpm / min

[0091] - Post-optics: Soler slit: 2.5°; Nickel filter: 0.0125mm; PSD: 4°. 1D detector (current value).

[0092] Qualitative analysis of diffraction patterns can be performed using the EVA software and the ICDD2016 database, followed by quantitative analysis using the HighScore Plus software based on Rietveld refinement.

[0093] The volume percentage and diameter of grains in α or β crystalline sintered materials can be determined by analyzing images obtained from electron backscatter diffraction (EBSD) observations. This setup can consist, for example, of a scanning electron microscope (SEM) equipped with an EBSD detector and a spectroscopic method with energy-dispersive X-ray spectroscopy (EDX). The EBSD and EDX detectors are controlled by the ESPRIT software (version 2.1). Available software can be used to acquire images with high crystallographic contrast and / or high density contrast.

[0094] The equivalent diameter of a grain corresponds to the diameter of a disk having the same surface area as the grain as observed along a cut plane of the material. The volume distribution of different equivalent diameters of the grain can be well represented using different sections of the material based on at least two perpendicular planes, and the median equivalent diameter (or D) of the grain can be derived by volume. 50 (Percentile). In this application, the volume percentage of sintered grains constituting the material is expressed relative to the volume of the material excluding its porosity.

[0095] The median equivalent diameter of a grain is equivalent to the diameter that divides the grain into first and second equal groups, which contain only grains with equivalent diameters that are respectively greater than or less than the median diameter.

[0096] The volume of the optional intergranular phase can also be calculated using the same method described above.

[0097] The total porosity (or total pore volume) of the material according to the invention is equivalent to the sum of the volumes of closed and open pores divided by the material volume. It is calculated as a percentage of the bulk density measured according to ISO 18754 to the absolute density measured according to ISO 5018.

[0098] The median diameter (or median “size”) of the particles constituting a powder can be given by characterization of the particle size distribution, particularly by a laser particle size analyzer. Characterization of particle size distribution is typically performed using a laser particle size analyzer according to ISO 13320-1. A laser particle size analyzer can be, for example, the Partica LA-950 from HORIBA. For the purposes of this specification and unless otherwise stated, the median diameter of a particle refers to the diameter of the particles found to be smaller than 50% by mass of the total. The “median diameter” or “median size” of a group of particles, particularly a group of powders, is referred to as D. 50 Percentile refers to the size at which particles are divided into first and second groups of equal volume, and these first and second groups contain only particles with sizes that are respectively larger or smaller than the median size.

[0099] The term "β-type silicon carbide powder" should be understood to refer to powder in which the 3C or cubic crystal form accounts for more than 95% of the silicon carbide mass. The α-type of SiC is mainly hexagonal or rhombohedral phase; 3H; 4H; 6H and 15R.

[0100] Specific surface area is measured by the BET (Brunauer Emmet Teller) method, as described, for example, in the Journal of American Chemical Society 60 (1938), pp. 309-316.

[0101] Unless otherwise stated, all percentages in this specification are mass percentages.

[0102] Exemplary Implementation

[0103] The following are non-limiting embodiments to enable the production of materials according to the invention, and of course, they do not limit the methods by which such materials may be obtained and the methods according to the invention, and comparative examples are given to show the advantages of the invention.

[0104] In all the following embodiments, the cylindrical ceramic body with a diameter of 30 mm and a thickness of 10 mm was initially produced by pouring slurry from the following raw materials into a plaster mold according to different formulations reported in Table 1 below:

[0105] 1) The powder is primarily composed of β-crystalline silicon carbide particles with a bimodal distribution. The first peak is located at 0.3 μm, and the second peak is approximately twice the height of the first, with its highest point at 3 μm. This is based on the non-cumulative size distribution measured by a laser particle size analyzer. The median diameter of the bimodal powder is 1.5 μm. This SiC powder has the following elemental mass levels:

[0106] Sc+Y+La+Ce+Pr+Nd+Pm+Sm+Eu+Gd+Tb+Dy+Ho+Er+Tm+Yb+Lu<0.5%

[0107] Nitrogen (N) < 0.2%;

[0108] Na+K+Ca+Mg<0.2%;

[0109] Aluminum (Al) <0.1%

[0110] Iron (Fe) < 0.05%;

[0111] Titanium (Ti) < 0.05%;

[0112] Molybdenum (Mo)<0.05%;

[0113] Its carbon, silicon dioxide, and free silicon contents are less than 2.0%, 1.0%, and 0.1%, respectively. Its β-SiC phase content is greater than 95%.

[0114] 2) Silicon carbide powder that is basically in the α-crystal form.

[0115] Its αSiC content is greater than 95% by mass. Its carbon, silicon dioxide, and free silicon contents are less than 0.2%, 1.5%, and 0.1%, respectively.

[0116] 3) Timcal provides C65 grade carbon black powder with a BET specific surface area of ​​62m². 2 / g.

[0117] 4) HCStarck provides HD-15 grade boron carbide B4C powder with a median diameter of 0.8 μm.

[0118] 5) Nanografi provides aluminum nitride powders of the following grades, with a median diameter of 0.06 μm.

[0119] The resulting granules were dried in air at 50°C. The granules from Examples 1 and 2 (comparative) were sintered in a furnace under argon and N2 atmospheres at 2150°C for 2 hours without pressure. The granules from Examples 3 and 4 (according to the invention) and Example 5 (comparative) were loaded into an apparatus for SPS sintering at 2000°C under a dual nitrogen atmosphere and a load of 85 MPa.

[0120] Unlike Examples 4 and 5, aluminum nitride powder was used instead of B4C powder, and sintering was carried out in a vacuum. Unlike Example 1, the starting powder in Example 7 was essentially β powder, and sintering was performed under the same conditions as in Example 6, in a vacuum and under pressure.

[0121] The total porosity of the sintered component is calculated by the difference between 100 and the ratio expressed as a percentage of the bulk density measured according to ISO 18754 and the absolute density measured according to ISO 5018.

[0122] Free silica (SiO2) content was measured by HF etching. Free carbon, oxygen, and nitrogen content were measured by LECO. Other elements were measured by X-ray fluorescence and ICP.

[0123] Free silicon was measured by controlling and then titrating with aqua regia. The percentage of β-type SiC and the ratio of β-type to α-type SiC were determined by X-ray diffraction analysis according to the above method.

[0124] The volume percentage and diameter of sintered materials with α or β crystal forms were determined by analyzing images obtained from EBSD observations.

[0125] The device consists of a scanning electron microscope (SEM) equipped with a Brukere-FlashHR+EBSD detector (with an FSE / BSE Argus imaging system) and an effective surface area of ​​10 mm². 2 Bruker A 4010 EDX detector was used. The EBSD detector was mounted on one of the rear ports of a FEI Nova NanoSEM 230 scanning electron microscope equipped with a field emission gun tilted at an angle of 10.6° relative to the horizontal plane to enhance both the EBSD and EDX signals. Under these conditions, the optimal working distance WD (i.e., the distance between the SEM electrode and the sample analysis area) was approximately 13 mm. The EBSD and EDS detectors were controlled by the ESPRIT software (version 2.1). FSE images (with high crystallographic contrast) and / or BSE images (with high density contrast) were acquired using an Argus system, with the EBSD camera positioned at a distance DD (sample detector distance) of 23 mm to minimize sensitivity to sample morphology. EBSD measurements were performed in point scan and / or plotting modes. For this purpose, the EBSD camera was positioned at a distance DD of 17 mm to increase the collected signal.

[0126] The equivalent diameter of a grain is equivalent to the diameter of a disk having the same surface area as the grain as observed along a cut plane of the material. By observing different sections of the material along at least two perpendicular planes, the distribution of different equivalent diameters of grains within the material volume can be determined, and the median equivalent diameter of the grain can be derived by volume.

[0127] The characteristics and performance obtained according to Examples 1 to 7 are given in Table 1 below.

[0128] Table 1

[0129]

[0130]

[0131] ND = Undetectable; NM = Not measured

[0132] Embodiments of the present invention demonstrate that high-purity, very dense crystalline silicon carbide materials can be obtained according to a very specific method comprising mixing substantially β-crystalline silicon carbide (SiC) in the presence of carbon, wherein a moderate amount of sintering additives is added, and the sintering is carried out under pressure and in a pure nitrogen atmosphere. Examples 6 and 7 (comparative) show that, unlike the method according to the present invention, regardless of whether the sintering additives used provide nitrogen (Example 6) or not (Example 7), vacuum sintering cannot yield dense SiC materials, i.e., with a porosity of less than 2%, or even less than 1%, and a median equivalent grain diameter of 1 to 10 micrometers.

Claims

1. A polycrystalline silicon carbide sintered material, comprising silicon carbide grains with a median equivalent diameter of 1 to 10 micrometers, wherein the total porosity of the material is less than 2% of the material volume, and the mass content of silicon carbide (SiC) excluding free carbon is at least 99%, wherein the mass ratio of the content of SiC having β-type crystal form to the content of SiC having α-type crystal form in the material is less than 2, and the material has the following elemental composition by mass: - Less than 0.5% of silicon in forms other than SiC, - Less than 2.0% of carbon in forms other than SiC, and - A total of 0.1% to 0.7% of at least one element selected from Al, B, Fe, Ti, Cr, Mg, Hf, and Zr. - Less than 0.5% oxygen (O), and - Elements with a total percentage less than 0.5% include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. - Less than 0.5% of alkali metal elements, and - Less than 0.5% of alkaline earth metals, and - 0.05 to 1% nitrogen (N), - Add other elements to bring the total to 100%.

2. The material according to claim 1, wherein the material has less than 1.5% carbon in forms other than SiC.

3. The material according to claim 1, wherein the material has a total content of 0.1% to 0.7% of at least one element selected from Zr, Ti, Hf, and B.

4. The material according to claim 1, wherein the material comprises more than 1% β-type SiC by weight relative to the total mass of the crystalline phases in the material.

5. The material of claim 1, wherein more than 90% of the grains have an equivalent diameter of 1 to 10 micrometers based on the volume of the material other than its porosity.

6. The material according to claim 1, wherein, based on the mass of the material: - The elemental mass content of nitrogen (N) is 0.05% to 0.5%.

7. The material according to claim 1, wherein the boron (B) content, by weight of the material, is greater than 0.1% and less than 0.7%.

8. The material according to claim 1, wherein the mass ratio of the SiC content of β-crystal SiC to the SiC content of α-crystal SiC in the material is less than 1.

9. The material according to claim 1, wherein the silicon carbide grains constitute at least 98% of the mass of the material, and the remainder consists of a residual intergranular phase comprising the elements Si and C.

10. The material according to claim 1, wherein the silicon carbide grains constitute at least 99% of the mass of the material, and the remainder consists of a residual intergranular phase comprising the elements Si and C.

11. The material according to claim 9 or 10, wherein the remainder comprises a residual intergranular phase composed of elements Si and C.

12. The material according to claim 1, wherein more than 90% by volume of the α-crystalline silicon carbide grains have an equivalent diameter of less than 10 micrometers.

13. A method for manufacturing polycrystalline silicon carbide sintered material according to any one of claims 1-12, comprising the following steps: a) Preparation of mineral raw materials, which, by mass, comprise: - At least 95% of the silicon carbide particles are in powder form, with a median size of 0.1 to 5 micrometers, and the SiC mass content is greater than 95%, wherein the β-crystal form accounts for greater than 90% of the total mass of silicon carbide. - At least one solid-state sintering additive comprising an element selected from aluminum, boron, iron, titanium, chromium, magnesium, hafnium, or zirconium, wherein the amount of said element is such that its contribution accounts for 0.1% to 0.8% of the total mass of the silicon carbide particles. - A carbon source of 0.5% to 3% with an elemental carbon content (C) greater than 99% by mass. b) Forming the raw materials into preforms. c) Solid-state sintering of the preform in a nitrogen atmosphere at a pressure greater than 60 MPa and a temperature greater than 1800 °C and less than 2100 °C.

14. The method of claim 13, wherein a) a mineral raw material is prepared, comprising at least 97% by weight silicon carbide particles in powder form.

15. The method according to claim 13, wherein the silicon carbide particles have a SiC mass content greater than 97%, and wherein the β crystal form accounts for greater than 95% of the total mass of silicon carbide.

16. The method according to claim 13, wherein at least one solid-phase sintering additive is in powder form.

17. The method of claim 13, wherein at least one solid-state sintering additive comprises an element selected from aluminum, boron, iron, titanium, chromium, magnesium, hafnium, or zirconium, with a purity greater than 98% by mass.

18. The method of claim 13, wherein the carbon source is in the form of uncrystallized or amorphous graphite or carbon powder, wherein the diameter is less than 1 micrometer.

19. The method of claim 13, wherein b) the raw material is formed into a preform by casting.

20. The method of claim 13, wherein the nitrogen atmosphere is a dinitrogen atmosphere.

21. The method according to claim 13, wherein the mass content of free carbon in the silicon carbide particle powder is less than 2%.

22. The method according to claim 13, wherein the mass content of free silicon dioxide in the silicon carbide particle powder is less than 1%.

23. The method according to claim 13, wherein the mass content of free silicon in the silicon carbide particle powder is less than 0.5%.

24. The method according to claim 13, wherein the sum of the elemental contents of aluminum (Al), alkali metals, alkaline earth metals, and rare earth metals containing at least one element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu in the silicon carbide particle powder is less than 0.5%.

25. The method of claim 13, wherein the element contained in the sintering additive is boron.

26. The method of claim 13, wherein the solid-state sintering step of the preform is performed by SPS ("spark plasma sintering").

27. An apparatus comprising the material according to any one of claims 1 to 12, the apparatus being selected from at least one of: turbines, pumps, valves, heat exchangers, devices for recovering heat, solar absorbers, furnace refractory coatings, cooking surfaces, cutting tools, brake pads, radomes, substrates for active layer deposition in the optical and / or electronic industries, heating elements, temperature sensors, pressure sensors, igniters, and magnetoresistors.

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