Dense sintered silicon carbide material with extremely low electrical resistivity

Abstract

Disclosed is a polycrystalline sintered ceramic material having very low electrical resistivity comprising, by weight: - More than 95% silicon carbide (SiC), - less than 1.5% silicon in forms other than SiC; - less than 2.5% carbon in a form other than SiC; - less than 1% oxygen (O), - less than 0.5% Aluminum (Al), - Less than 0.5% of the sum of the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; - less than 0.5% alkali elements; - less than 0.5% alkaline earths, - 0.05-1% nitrogen (N), - other elements that form the complement to 100% wherein the grains of said material have a median equivalent diameter of 0.5 to 5 micrometers, the SiC alpha (α) / SiC beta (β) mass ratio is less than 0.1, and the total porosity is less than 15% by volume of said material.

Description

[Technical Field]

[0001] This invention relates to a dense material based on silicon carbide (SiC), which can be used, in particular, for its high conductivity. [Background technology]

[0002] Silicon carbide materials have long been known for their high hardness, chemical inertia, heat resistance, mechanical resistance, and thermal conductivity. This makes them a candidate for applications such as: cutting tools or machining tools; turbine components or pump elements subject to high wear; pipe valves for carrying corrosive products; supports and membranes for filtering or decontaminating gases or liquids; oven linings and countertops; coatings or materials for heat exchangers and solar heat absorbers, thermochemical treatment of reactors, especially for etching, or substrates, temperature sensors or heating resistors intended for the electronics industry; high temperature or high pressure sensors or sensors for extremely harsh environments; igniters or magnetic susceptors with higher oxidation resistance than those made of graphite; and even specific applications such as mirrors or other optical devices.

[0003] However, these types of materials have variable electrical resistivity, or even high electrical resistivity (approximately 0.1 to several tens of ohms / cm at 20°C), which imposes limitations on their use. For example, to enhance the use of this material, particularly as an igniter, various hot-sintered silicon carbide materials with added aluminum, boron, or silicon nitride and / or molybdenum disilide have been proposed in U.S. Patent No. 3,974106, U.S. Patent No. 5,045237, or U.S. Patent No. 5,085804. However, these materials have low silicon carbide content or high porosity, which impairs their performance, and in particular their thermal conductivity or high-temperature properties.

[0004] More recently, the effect of the addition of rare earth metals related to AlN on the electrical resistivity of SiC sintered bodies was studied in the paper "Electrical resistivity of silicon carbide ceramics sintered with 1 wt% aluminum nitride and rare earth oxide" by Young-Wook Kim et al. in the Journal of the European Ceramic Society 32 (2012) 4427-4434. The starting mixture essentially contains beta-crystalline or cubic SiC, forming additives including siloxane and phenolic resin, and less than 1 wt% by mass of rare earth powder and AlN powder. This mixture is dried, molded by unidirectional pressing, and then cured at 200°C, thereby obtaining a workpiece that is pre-treated at 1450°C, followed by sintering at 2050°C under a nitrogen atmosphere at 20 MPa. The resulting material has a relative density of over 95% and a density of 1.5 × 10⁻⁶. -4 ~2.9×10 -2 The resistivity is in ohms·m, i.e., 15 to 290 milliohms·cm, depending on the rare earth elements added.

[0005] However, as explained by Y. Taki et al. in their paper "Electrical and thermal properties of nitrogen-doped SiC sintered body" in the Journal of the Japan Society of Powder and Powder Metallurgy, Vol. 65, No. 8, 2018, adding Al in the AlN form is advantageous for densification but results in a liquid phase that is detrimental to high-temperature mechanical properties.

[0006] Solid-phase sintering techniques that do not rely on the liquid phase, such as pressureless sintering from the addition of boron and carbon, have been known for some time, as described, for example, in U.S. Patent No. 4004934. More recently, as shown in the paper "Pressureless sintering of beta silicon carbide nanoparticles" by A. Malinge et al. in the Journal of the European Ceramic Society 32 (2012) 4393-4400, using submicron-sized beta-crystalline silicon carbide starting powder makes it possible to achieve a relative density of approximately 90%. However, the sintering temperature required for this high density inevitably leads to the formation of an alpha-silicon carbide phase, which increases electrical resistivity, as explained in the same paper. [Overview of the project] [Problems that the invention aims to solve]

[0007] Therefore, an object of the present invention is to provide a sintered SiC material that has a low electrical resistivity, i.e., less than 100 milliohms·cm, preferably less than 50 milliohms·cm, and also has high mechanical and thermal properties even at high temperatures. [Means for solving the problem]

[0008] The applicant's research, described below, has demonstrated that a material with optimal physical / chemical composition can be obtained, without the use of additives based on aluminum elements or rare earth metals that are harmful at high temperatures, while maintaining the lowest possible porosity (less than 10% by volume) and achieving extremely low electrical resistivity (less than 50 milliohms / cm at room temperature (20°C)). This was achieved through appropriate selection of starting materials and specific methods, which also minimize, or even avoid, the formation of alpha-form SiC and any liquid phase formation at grain boundaries. In fact, the applicant has found that these two factors can adversely affect conductivity. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is an image of the polished cross-section of the sintered material of Example 3 according to the present invention, taken using a scanning microscope. [Modes for carrying out the invention]

[0010] Accordingly, according to a first aspect of the present invention, the present invention relates to a polycrystalline ceramic material comprising sintered grains having a median equivalent diameter of 0.5 to 5 micrometers, wherein the material contains more than 95% silicon carbide (SiC) by mass ratio, preferably more than 97% silicon carbide, and has the following elemental composition by mass ratio: - Less than 1.5% silicon, a form other than SiC, - Less than 2.5% of carbon is a form other than SiC, and - Less than 1%, preferably less than 0.75%, more preferably less than 0.5% oxygen (O), and, - Less than 0.5% aluminum (Al), and, - The total amount of the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu is less than 0.5%, and, - Less than 0.5% alkali elements, and, - Alkaline earth elements of less than 0.5%, and - Preferably less than 0.50% boron (B), more preferably less than 0.2% boron, - 0.05~1% nitrogen (N), - Other elements that form the complement to 100%, Here, - The mass ratio of the alpha-crystalline (α) SiC content to the beta-crystalline (β) SiC content of the above material is less than 0.1, preferably less than 0.05, and - The total porosity accounts for less than 15%, preferably less than 12%, and more preferably less than 10% by volume of the above material.

[0011] The elemental composition described above by elements such as Si, C, O, Al, etc. of course means an addition to silicon carbide, that is, an addition to silicon carbide of more than 95% by mass (preferably more than 97% by mass) present in the above material.

[0012] Silicon, which is in a form different from SiC, may exist especially in the form of free silica and / or free silicon (metallic silicon).

[0013] Carbon, which is in a form different from SiC, may exist especially in the form of free carbon.

[0014] According to other optional but advantageous additional features of the above material, they are as follows: - The material contains more than 0.1%, preferably more than 0.5%, of silicon in a form different from SiC, especially in the form of free silica and / or free (metallic) silicon. - The material contains more than 0.1%, preferably more than 0.5%, of carbon in a form different from SiC, especially in the form of free carbon. - The material contains more than 0.1%, preferably more than 0.5%, of oxygen (O). - The material does not contain silicon in a form different from SiC except in the form of inevitable impurities. - The material does not contain carbon in a form different from SiC except in the form of inevitable impurities.

[0015] The material does not contain oxygen (O), aluminum (Al), Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, alkali metals, alkaline earth elements in a form different from inevitable impurities.

[0016] The material does not contain boron except in the form of inevitable impurities.

[0017] The material does not contain other elements except in the form of inevitable impurities. - The total elemental content of sodium (Na) + potassium (K) + calcium (Ca) is less than 0.5% cumulatively of the mass of the above material. - The elemental mass content of aluminum (Al) accounts for less than 0.3% of the mass of the above material. - The total elemental content of alkali, alkaline earth, aluminum, and rare earth is less than 2% cumulatively of the mass of the above material, preferably less than 1%, more preferably less than 0.5%. - The elemental content of boron (B) is less than 0.2% of the mass of the above material and / or more than 0.02% of the mass of the above material. - The elemental content of zirconium (Zr) is less than 1.0% of the mass of the above material, preferably less than 0.8%, preferably less than 0.5%. - The elemental content of zirconium (Zr) is more than 0.02% of the mass of the above material, preferably more than 0.05%, preferably more than 0.1%, preferably more than 0.2%. - The elemental content of molybdenum (Mo) is less than 0.2% of the mass of the above material, preferably less than 0.1% of the mass of the above material. - The elemental content of titanium (Ti) is less than 1.0% of the mass of the above material, preferably less than 0.8%, preferably less than 0.5%. - The elemental content of titanium (Ti) is more than 0.02% of the mass of the above material, preferably more than 0.05%, preferably more than 0.1%, preferably more than 0.2%. - The elemental content of hafnium (Hf) is less than 1.0% of the mass of the above material, preferably less than 0.8%, preferably less than 0.5%. - The elemental content of hafnium (Hf) is more than 0.02% of the mass of the above material, preferably more than 0.05%, preferably more than 0.1%, preferably more than 0.2%. - The total elemental content of Zr, Hf, and Ti is 0.05% - 1%. - The elemental content of nitrogen is more than 0.1%. - The elemental content of nitrogen is less than 0.8% of the mass of the above material, preferably less than 0.7%, preferably less than 0.5%. - The elemental mass content of iron (Fe) accounts for less than 0.5% of the mass of the above material. - Silicon, in a form other than silicon carbide (SiC), accounts for less than 1% of the mass of the above material. - Carbon, in a form other than silicon carbide (SiC), accounts for less than 2% of the mass of the above material. - The mass content of free or residual carbon in the above material is less than 1.5%, preferably less than 1.0%. - The mass content of free or residual silica in the above material is less than 1.5%, preferably less than 1.0%, and preferably less than 0.5%. - The mass content of free or residual silicon in the above material is less than 0.5%, preferably less than 0.1%. - Oxygen accounts for less than 0.4%, preferably less than 0.3%, of the mass of the above material. - SiC accounts for more than 97%, preferably more than 98%, of the mass of the above material. - SiC in beta crystalline form (β) preferably accounts for more than 90% of the mass of the crystalline phase of the above material. - The equivalent diameter of silicon carbide grains in alpha crystalline form is less than 10 micrometers. - By volume, more than 90%, preferably more than 95%, of the particles have an equivalent diameter of 0.5 to 5 microns (micrometers), preferably 0.5 to 3 microns (micrometers). - Of the above materials, excluding porosity, more than 90%, preferably more than 93%, and more preferably more than 95% of the grains are silicon carbide grains in beta crystalline form. The term "silicon carbide grains in beta crystalline form" means grains in which the mass content of beta SiC is more than 93%, preferably more than 95%, and preferably more than 97%. - The grains of the above material, whose equivalent diameter is 0.5 to 5 microns (micrometers), are essentially in a beta crystalline form. - Silicon carbide grains in alpha crystalline form account for less than 10%, preferably less than 5%, of the material by volume, excluding its porosity. According to one embodiment, the material may contain at least 0.5% of silicon carbide grains in alpha crystalline form, excluding its porosity. - By volume, silicon carbide grains that are in alpha crystalline form, particularly more than 90%, preferably more than 95%, and even more preferably all, have an equivalent diameter of less than 5 micrometers, preferably less than 2 micrometers, or even less than 1 micrometer, and the growth of such grains is suppressed in accordance with the present invention in order to minimize the electrical resistivity of the above material. - In the material comprising the material according to the present invention, nitrogen is present within the grains by being inserted into the SiC crystal lattice. Furthermore, nitrogen is also present on the surface and grain boundaries of the constituent grains of the material, as is mainly the case with the elements Si and C. - The total porosity of the above material is less than 5%, preferably less than 4%, and more preferably less than 3%, by volume. - The median pore size of the above material is less than 2 micrometers. - The above material has an electrical resistivity of less than 50 milliohms·cm, preferably less than 30 milliohms·cm, and preferably less than 20 milliohms·cm, as measured at 20°C and atmospheric pressure.

[0018] In this specification, unless otherwise specified, all percentages are as follows: - The chemical composition or crystallographic composition is expressed as a mass ratio, and - Particle size or pore size is expressed as a volume ratio.

[0019] The present invention also relates to a method for producing the above material, comprising the following steps: (a) To produce a supply material that, in mass ratio, includes, preferably essentially consists of, the following: - A powder of silicon carbide particles having a median diameter of 0.1 to 5 micrometers, comprising at least 95% silicon carbide particles, in a beta crystalline form, with a silicon carbide content of at least 95% by mass, and - Preferably a sintering additive of less than 0.2%, preferably containing boron, and - Less than 3% carbon or carbon precursor, preferably amorphous or noncrystalline graphite or carbon powder, with a median diameter of less than 1 micrometer. - Less than 2% silicon or silicon precursor, preferably metal or amorphous silicon powder, preferably metallic, with a median diameter of less than 5 micrometers. (b) Forming the supplied raw material into the shape of a preform, preferably by casting. (c) Solid-phase sintering of the preform in a nitrogen atmosphere, preferably a dinitrogen atmosphere, at a pressure of over 60 MPa, preferably over 75 MPa, or even more preferably over 80 MPa, and at a temperature of over 1800°C and less than 2100°C.

[0020] The possible and limited additions of carbon or silicon precursors are intended to react with residual silicon or silica or residual carbon present in the silicon beta carbide powder, respectively, in order to form silicon carbide through the reaction.

[0021] The following are other optional and advantageous additional features of the above method: - The nitrogen mass content of the silicon carbide powder in beta crystalline form is greater than 0.1%, preferably less than 1%. - The specific surface area of ​​silicon carbide powder in beta crystalline form is 5 cm². 2 / g or more and / or 30cm 2 It is less than / g. - The elemental mass content of aluminum in the silicon carbide powder, which is in beta crystalline form, is less than 0.1%. - The total elemental mass content of Na+K+Ca+Mg in the silicon carbide powder in beta crystalline form is less than 0.2%. - The total elemental mass content of Sc+Y+La+Ce+Pr+Nd+Pm+Sm+Eu+Gd+Tb+Dy+Ho+Er+Tm+Yb+Lu in silicon carbide powder in beta crystalline form is less than 0.5%. - The silicon carbide powder is essentially in a beta crystalline form, i.e., the beta phase mass content is at least 95%, and the SiC mass content is over 99%. - The mass content of free or residual carbon in the silicon carbide powder, which is essentially in beta crystalline form, is less than 3%, preferably less than 2%, and preferably less than 1.5%. - The mass content of free or residual silica in the silicon carbide powder, which is essentially in beta crystalline form, is less than 2%, preferably less than 1.5%, and preferably less than 1%. - The mass content of free or residual silicon in the silicon carbide powder, which is essentially in beta crystalline form, is less than 0.5%, preferably less than 0.1%. - The total elemental mass content of contaminants or impurities, represented by elements or species other than silicon, free silica, or residual carbon, in silicon carbide powder that is essentially in beta crystalline form is less than 1%. - The above-mentioned silicon carbide particle powder has a total mass content of less than 3% free or residual carbon, less than 2% free or residual silica, less than 0.5% free or residual silicon, and less than 1 elemental mass content of contaminants or impurities. - The silicon carbide powder, which is essentially in a beta crystalline form, is bimodal and, when measured by laser granulometry, has two peaks, more preferably a first peak with a peak of 0.1 to 0.5 micrometers, and a second peak with a peak of 1 to 6 micrometers. - The specific surface area of ​​silicon carbide powder, which is essentially in beta crystalline form, is 5 cm². 2 / g~30cm 2 It is / g. - The supply material contains at least 0.05% solid-phase sintering additive, preferably zirconium and / or titanium and / or hafnium, wherein the additive is preferably a metal powder, oxide, nitride, carbide, boride, or fluoride of any one of these elements. Preferably, the supply material contains less than 1% of a sintering additive containing an element selected from zirconium and / or titanium and / or hafnium, wherein the additive is preferably a metal powder, oxide, nitride, carbide, boride, or fluoride of any one of these elements. The powder has a purity of more than 98% by mass, i.e., the total content of other elements is less than 2% by weight. - The supplied raw materials do not contain solid-phase sintering additives. The silicon carbide powder may be doped with at least one element from among zirconium and / or titanium and / or hafnium. - The supplied raw material contains at least 0.05% silicon or silicon precursor. - The supplied raw materials do not contain silicon or silicon precursors. - The supplied raw materials are, for example, in the form of aluminum nitride powder or aluminum powder, and do not contain the intentional addition of aluminum or aluminum precursors. - The supplied raw materials do not contain the intentional addition of rare earth elements or precursors that are any of the elements Sc+Y+La+Ce+Pr+Nd+Pm+Sm+Eu+Gd+Tb+Dy+Ho+Er+Tm+Yb+Lu. - The supplied raw materials contain at least 0.05% carbon or carbon precursors. - The supplied raw materials do not contain carbon or carbon precursors. - The median diameter of the sintered powder is less than 2 micrometers, preferably less than 1 micrometer. Preferably, it is boron carbide powder. - In one possible embodiment, the sintering additive contains the element zirconium. In one possible embodiment, the sintering additive is a powder of carbide, fluoride, or zirconium boride. - The supplied raw material contains at least 0.5%, preferably at least 1%, of a carbon precursor. - The supplied raw materials contain less than 0.5% silicon precursor, or even no silicon precursor at all. - The supplied raw materials may optionally contain less than 1% organic additives that essentially contain one of the elements C, O, H, N, or Si. For example, acrylic resins, PEG, siloxanes, vinyl, epoxy, phenol, polyurethane compounds or resins, alkyd derivatives, or glycerophthalic compounds may be suitable.

[0022] Any molding technique known to those skilled in the art may 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 may be adapted 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 to avoid contamination of the preform. These controlled precautions, for use by those skilled in the art, may also be applied to other steps of the method. Therefore, the mold or matrix used in sintering, including the preform, would preferably be made of graphite.

[0023] Hot pressing, hot isostatic pressing, or SPS (discharge plasma sintering) technology are particularly suitable. Preferably, pressure-assisted sintering is performed by SPS, which is a sintering process that induces heating by passing a direct current through the 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 furnace load. The nitrogen used in the sintering atmosphere in step (c) has a purity of greater than 99.99% by volume, or even greater than 99.999% by volume.

[0024] The present invention also relates to an apparatus comprising the material according to the present invention, wherein the apparatus is selected from the following: turbines, pumps, valves or fluid line systems; heat exchangers; solar heat absorbers or apparatus for heat recovery or light reflection; refractory coatings for furnaces; cooking surfaces; crucibles for melting metals or metalloids; anti-wear parts; cutting tools; brake pads or discs; coatings or supports for thermochemical treatment, such as etching; or substrates for active layer deposition for the optical and / or electronic industries; heating elements or resistors; temperature or pressure sensors; igniters; magnetic susceptors. Preferably, the apparatus is selected from the following: turbines, pumps, valves or fluid line systems; anti-wear parts; cutting tools; brake pads or discs; heating elements or resistors; temperature or pressure sensors; igniters; magnetic susceptors; substrates for active layer deposition for the optical and / or electronic industries.

[0025] definition

[0026] The following notations and definitions are given in connection with the foregoing description of the present invention: Polycrystalline materials are understood to be materials that have crystals with multiple or various crystal orientations. - In ceramic materials, the sintered grains form a unified whole that constitutes the essential mass of the material, and the grain boundary phases consist optionally of a ceramic phase and / or a metallic phase, or residual carbon, and preferably account 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 present invention is carried out essentially in a solid phase, that is, it is a sintering in which the level of additives added to enable sintering or optionally present impurities is such that it does not allow the formation of a liquid phase, but rather does not allow the grains to rearrange and thus bring them into contact with each other. The material obtained by solid-phase sintering is generally called a "solid-phase sintered body".

[0027] Sintering additives (often simply called “additives”) are understood herein to mean compounds conventionally known for facilitating and / or accelerating the reaction kinetics of sintering reactions. - Impurities are understood to mean unavoidable components that are introduced unintentionally and inevitably along with the raw materials, or that arise from reactions between components. Impurities are not necessary components, but merely acceptable ones. - The elemental chemical content of the sintered material, or the elemental chemical content of the powder used in the mixture of the above material manufacturing method, is measured according to the techniques well known in the art. In particular, the levels of elements such as Al, B, Ti, Zr, Fe, 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 level present, especially by ICP if the level is less than 0.5%, or even less than 0.2%, in accordance with ISO 21068-3:2008, on products that have been fired in air at 750°C until weight loss occurs.

[0028] The mass ratios of free silicon, free silica, free carbon, and SiC content are measured according to ISO 21068-2:2008. The oxygen and nitrogen content is determined by LECO according to ISO 21068-3:2008.

[0029] The polytype composition of SiC, and the presence of other phases in the sintered material, or other phases in the powder used in the mixture of the above material manufacturing method, are usually obtained by X-ray diffraction and Rietveld analysis. In particular, the percentages of alpha-SiC phase and beta-SiC phase, respectively, can be determined using a D8 Endeavor instrument manufactured by BRUKER, with the following configuration: - Acquisition: d5f80: 5°~80° at 2θ, 0.01° step, 0.34 sec / step, duration 46 minutes, - Front optics: Primary slope 0.3°, solar slit 2.5°, - Sample holder: Automatic cutter with a rotation speed of 5 rpm / min. - Rear optics: Solar slit: 2.5°; Nickel filter 0.0125mm; PSD: 4°. 1D detector (current value).

[0030] The diffraction pattern may be qualitatively analyzed using EVA software and the ICDD2016 database, and quantitatively analyzed according to Rietveld refinement using HighScore Plus software.

[0031] The volume percentage of sintered material grains in alpha or beta form, and their diameters, can be determined by analysis of images obtained from electron backscatter diffraction (EBSD) observations. The setup may consist, for example, a scanning electron microscope (SEM) equipped with an EBSD detector and a spectroscopic analyzer equipped with energy-dispersive X-ray spectroscopy (EDX). The EBSD and EDX detectors are controlled by the ESPRIT software (version 2.1). Images with high crystallographic contrast and / or high density contrast can be acquired using available software.

[0032] The equivalent diameter of a grain corresponds to the diameter of a disk with the same surface area as the surface area of ​​the grain observed along the cutting plane of the material. By using various cross-sections of the material along at least two perpendicular planes, a very good representation of the volume distribution of various equivalent diameters of grains can be obtained, and from there the median (or D) of the equivalent diameter of the grain can be determined by volume. 50 The percentile can be estimated. In this application, the volume percentage of sintered grains constituting the material is expressed as a relative value to the volume of the material excluding its porosity.

[0033] This median diameter (or D 50 A percentile grain corresponds to the diameter that divides the grain into first and second equal groups, where each of these first and second groups consists only of grains with an equivalent diameter greater than the median diameter, or only of grains with an equivalent diameter less than the median diameter.

[0034] The volume of any grain boundary phases present at will may be calculated using the same method as described above.

[0035] The total porosity (or total pore volume) of the material according to the present invention corresponds to the sum of the volumes of the closed pores and the open pores divided by the volume of the material. This is calculated according to a ratio expressed as a percentage of the apparent specific gravity measured according to ISO 18754 to the true density measured according to ISO 5018.

[0036] The median particle diameter (or median "size") of the particles constituting the powder can be obtained by characteristic evaluation of the particle size distribution, particularly by a laser particle size analyzer. The characteristic evaluation of the particle size distribution has conventionally been performed using a laser particle size analyzer conforming to the ISO 13320-1 standard. The laser particle size analyzer can be, for example, Partica LA-950 from HORIBA, Ltd.

[0037] For the purposes of this specification, unless otherwise particularly noted, the median diameter of the particles respectively indicates the diameter of the following particles where 50 mass% of the population is found. The "median diameter" or "median size" of a collection of particles is called the D 50 percentile, that is, the size that divides the particles into first and second populations of equal volume, and these first and second populations have only particles with an equivalent diameter greater than the median diameter or only particles with an equivalent diameter less than the median diameter, respectively.

[0038] The specific surface area is measured by the B.E.T. (Brunauer Emmet Teller) method described, for example, on pages 309-316 of Journal of American Chemical Society 60 (1938).

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

[0040] Unless otherwise specified, all percentages in this specification are mass percentages.

[0041] Figure 1 is an image of the polished cross-section of the sintered material of Example 3 according to the present invention, taken using a scanning microscope. [Examples]

[0042] Exemplary Embodiments

[0043] The following are non-limiting embodiments that enable the production of materials according to the present invention, but this does not, of course, limit the methods that enable the production of such materials or the methods according to the present invention.

[0044] Comparative examples are shown below to illustrate the advantages of the present invention.

[0045] In all of the following examples, cylindrical ceramic bodies having a diameter of 30 mm and a thickness of 10 mm were first manufactured by casting a slurry (slip) into a plaster mold from the following raw materials according to the various formulations reported in Table 1 below: (1) Powder of silicon carbide (SiC) particles in beta crystalline form. According to the non-cumulative particle size distribution measured by a laser particle size analyzer, it has a bimodal distribution with a first peak at 0.3 micrometers and a second peak at 3 micrometers, substantially twice the height of the first peak. The median diameter of the bimodal powder is 1.5 micrometers. This 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%; - Zr<0.1; Hf<0.1 The carbon, silica, and free silicon content are less than 2.0%, less than 1.0%, and less than 0.1%, respectively. The mass content of the beta-SiC phase is over 95%. (2) Provided by Timcal in grade C65, 62m 2 Carbon black powder having a BET specific surface area of ​​1 / g. (3) Boron carbide powder supplied by H.C. Starck in grade HD-15, having a median diameter of 0.8 micrometers. (4) Zirconia powder supplied by Saint-Gobain Zirpro in grade CY3Z-RA, having a median diameter of 0.3 micrometers. (5) Titanium oxide powder supplied by Sigma-Aldrich, having a median diameter of 0.1 micrometers. (6) Aluminum nitride powder supplied by Nanografi, having a median diameter of 0.06 micrometers.

[0046] The pellets produced in this manner are dried in air at 50°C. The pellets of Examples 1 and 2 (Comparative Examples) are sintered in an argon oven at a temperature of 2150°C for 2 hours without pressure or load. The pellets of Example 3 (according to the present invention) and Example 5 (Comparative Example) are loaded into an SPS sintering apparatus at 2000°C under a load of 85 MPa (megapascals) in a dinitrogen atmosphere. Unlike Example 3, the sintering of the pellets of Example 4 (Comparative Example) is carried out under vacuum. Example 6 according to the present invention is carried out under the same conditions as Example 5, but the boron carbide powder is replaced with zirconia powder, as in Example 8 (also according to the present invention). Unlike Example 6, in Example 7 (according to the present invention), the addition is in the form of titanium oxide powder. In Examples 9 and 10 (Comparative Examples), unlike Example 7, the addition consists of aluminum nitride powder. The sintering of Examples 9 and 10 is the same as the sintering of Example 7 (pressure-assisted sintering, in N2) and Example 4 (pressure-assisted sintering, under vacuum), respectively.

[0047] The total porosity of the obtained material is calculated by taking the difference between 100 and the ratio of the apparent density, measured according to ISO 18754, to the true density, measured according to ISO 5018. The free silica content (SiO2) is measured by HF attack. The free carbon, oxygen, and nitrogen content is measured by the LECO method. The free silicon content is measured by titration after control using aqua regia. Other elemental levels are measured by X-ray fluorescence and ICP. The percentage of beta-form SiC and the ratio of crystalline form β / α SiC are determined by X-ray diffraction analysis according to the method described above.

[0048] The electrical resistivity is measured at four points on a sample with a diameter of 20-30 mm and a thickness of 2.5 mm, according to the Van der Pauw method, at room temperature (20°C).

[0049] The volume percentage and diameter of sintered material grains in alpha or beta form were determined by analysis of images obtained from EBSD observations. The equipment included a Bruker e-FlashHR+ EBSD detector with an FSE / BSE Argus imaging system, and a 10mm 2The system consists of a scanning electron microscope (SEM) equipped with a Bruker XFlash 4010 EDX detector having an active surface area. The EBSD detector is mounted on one of the rear ports of the FEI Nova NanoSEM 230 scanning electron microscope, which has a field emission gun with a tilt angle equal to 10.6° 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 sample analysis area) is approximately 13 mm. The EBSD and EDS detectors are 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 the Argus system by positioning the EBSD camera at a distance DD (sample detector distance) of 23 mm to make it relatively less susceptible to the effects of sample topography. EBSD measurements were performed in point scan and / or mapping mode. For this purpose, the EBSD camera was positioned at a distance DD of 17 mm to increase the acquired signal.

[0050] The equivalent diameter of a grain corresponds to the diameter of a disk with the same surface area as the grain, observed along the cutting plane of the material. By observing various cross-sections of the material along at least two vertical planes, the distribution of various equivalent diameters of the grains in the volume of the material can be determined, and from this, the median equivalent diameter of the grains can be estimated from the volume.

[0051] The characteristics and properties obtained according to Examples 1-6 are shown in Table 1 below.

[0052] [Table 1]

[0053] NM = Not measured

[0054] A comparison of Examples 3 and 6-8 of the present invention with other comparative examples demonstrates that, under the precise and unique conditions of the present invention, it is possible to obtain crystalline silicon carbide materials that are not very porous and have little or no electrical resistivity, starting from a pure mixture of beta-form SiC, very little or no sintering additives and / or carbon, and pressure-assisted sintering in the presence of a nitrogen atmosphere. Examples 9 and 10 show that the addition of aluminum results in much higher resistivity, regardless of the type of sintering. This disclosure includes the following aspects: <Aspect 1> A polycrystalline ceramic material comprising sintered grains having a median equivalent diameter of 0.5 to 5 micrometers, wherein the material contains more than 95% silicon carbide (SiC) by mass and has the following elemental composition by weight: - Silicon in a form other than silicon carbide, less than 1.5% - Less than 2.5% of carbon, which is a form other than silicon carbide. - Less than 1% oxygen (O), - Less than 0.5% aluminum (Al), - Total amount of elements less than 0.5%: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu - Less than 0.5% alkali elements, - Less than 0.5% alkaline earth elements, - 0.05~1% nitrogen (N), - Other elements that form the complement to 100%, Here: - The mass ratio of silicon carbide content in alpha crystal form (α) to silicon carbide content in beta crystal form (β) of the material is less than 0.1. - The total porosity is less than 15% by volume of the above material. Polycrystalline ceramic material. <Aspect 2> The polycrystalline ceramic material according to Embodiment 1, having the following elemental composition by weight: - Less than 0.5% oxygen (O) and / or, - Boron (B) content of less than 0.2%. <Aspect 3> The polycrystalline ceramic material according to embodiment 2, wherein the total elemental content of sodium (Na) + potassium (K) + calcium (Ca) is less than 0.5% cumulatively of the mass of the material. <Aspect 4> A polycrystalline ceramic material according to any one of embodiments 1 to 3, wherein the elemental nitrogen content is less than 0.5% of the mass of the material. <Aspect 5> A polycrystalline ceramic material according to any one of embodiments 1 to 4, wherein the elemental content of iron (Fe) is less than 0.5% of the mass of the material. <Aspect 6> A polycrystalline ceramic material according to any one of embodiments 1 to 5, wherein the elemental content of an element selected from the group consisting of zirconium, titanium, and hafnium is greater than 0.02% and less than 1%. <Aspect 7> A polycrystalline ceramic material according to any one of embodiments 1 to 6, wherein the cumulative elemental content of Zr, Hf, and Ti is 0.05% to 1%. <Aspect 8> The polycrystalline ceramic material according to any one of embodiments 1 to 7, wherein the silicon carbide constitutes more than 97%, preferably more than 98%, of the mass of the material. <Pattern 9> A polycrystalline ceramic material according to any one of embodiments 1 to 8, wherein, in the volume ratio of the material excluding its porosity, more than 90% of the grains have an equivalent diameter of 0.5 to 5 micrometers. <Aspect 10> A polycrystalline ceramic material according to any one of embodiments 1 to 9, wherein, by volume ratio excluding porosity, more than 90% of the grains of the material are silicon carbide grains in a beta crystalline form. <Aspect 11> A polycrystalline ceramic material according to any one of embodiments 1 to 10, wherein the equivalent diameter of the silicon carbide grains in alpha crystalline form is less than 10 micrometers. <Aspect 12> A polycrystalline ceramic material according to any one of embodiments 1 to 11, having an electrical resistivity of less than 50 milliohms·cm as measured at 20℃ and atmospheric pressure. <Aspect 13> A method for manufacturing a polycrystalline sintered ceramic material according to any one of embodiments 1 to 12, comprising the following steps: (a) To produce a supply material containing the following in mass ratio: - A powder of silicon carbide particles having a median size of 0.1 to 5 micrometers, comprising at least 95%, in a beta crystalline form, with a silicon carbide content of at least 95% by mass, and - Preferably a solid-phase sintering additive of less than 0.2%, wherein the additive advantageously contains boron, and - Less than 3% carbon or carbon precursors with a median diameter of less than 1 micrometer. - Less than 2% silicon or silicon precursor with a median diameter of less than 5 micrometers. (b) Forming the supplied raw material into the shape of a preform, preferably by casting. (c) Solid-phase sintering of the preform in a nitrogen atmosphere at a pressure exceeding 60 MPa and at a temperature exceeding 1800°C but less than 2100°C. <Aspect 14> The manufacturing method according to embodiment 13, wherein the silicon carbide particle powder has a total mass content of less than 3% free or residual carbon, less than 2% free or residual silica, less than 0.5% free or residual silicon, and less than 1% elemental mass content of contaminants or impurities. <Aspect 15> The manufacturing method according to embodiment 14, wherein the supplied raw material preferably contains less than 0.2% of a solid-phase sintering additive containing boron. <Aspect 16> The manufacturing method according to any one of embodiments 13 to 15, wherein the supply raw material contains at least 0.05% of a solid-phase sintering additive, preferably zirconium and / or titanium and / or hafnium, and the additive is preferably a metal powder, oxide, nitride, carbide, boride, or fluoride of any one of these elements. <Aspect 17> The manufacturing method according to any one of embodiments 13 to 16, wherein the supplied raw material does not contain silicon or a silicon precursor and / or does not contain aluminum or an aluminum precursor. <Aspect 18> Apparatus comprising the material described in any of embodiments 1 to 12, wherein the apparatus is selected from the following: apparatus: turbines, pumps, valves or fluid line systems; heat exchangers; solar heat absorbers or devices for heat recovery or light reflection; refractory coatings for furnaces; cooking surfaces; crucibles for melting metals or metalloids; wear-resistant parts; cutting tools; brake pads or discs; coatings or supports for thermochemical treatment; or substrates for active layer deposition for the optical and / or electronic industries; heating elements or resistors; temperature or pressure sensors; igniters; magnetic susceptors.

Claims

1. A polycrystalline ceramic material comprising sintered grains having a median equivalent diameter of 0.5 to 5 micrometers, wherein the polycrystalline ceramic material contains more than 95% silicon carbide (SiC) by mass and has the following elemental composition by weight: - Silicon in a form other than silicon carbide, less than 1.5% - Less than 2.5% of carbon, which is a form other than silicon carbide. - Less than 1% oxygen (O), - Less than 0.5% aluminum (Al), - Total amount of elements less than 0.5%: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu - Less than 0.5% alkali elements, - Alkaline earth elements of less than 0.5% - 0.05 to 1% nitrogen (N), - Other elements that form the complement to 100%, Here: - The mass ratio of silicon carbide content in alpha crystal form (α) to silicon carbide content in beta crystal form (β) of the polycrystalline ceramic material is less than 0.

1. - The total porosity is less than 15% by volume of the above polycrystalline ceramic material. The elemental content of an element selected from the group consisting of zirconium, titanium, and hafnium is greater than 0.02% and less than 1%. Here, the cumulative elemental content of Zr, Hf, and Ti is 0.05% to 1%. Polycrystalline ceramic material.

2. The polycrystalline ceramic material according to claim 1, having the following elemental composition by weight: - Less than 0.5% oxygen (O) and / or, - Boron (B) content of less than 0.2%.

3. The polycrystalline ceramic material according to claim 2, wherein the total elemental content of sodium (Na) + potassium (K) + calcium (Ca) is less than 0.5% of the mass of the polycrystalline ceramic material.

4. The polycrystalline ceramic material according to any one of claims 1 to 3, wherein the elemental nitrogen content is less than 0.5% of the mass of the polycrystalline ceramic material.

5. The polycrystalline ceramic material according to any one of claims 1 to 3, wherein the elemental content of iron (Fe) is less than 0.5% of the mass of the polycrystalline ceramic material.

6. The polycrystalline ceramic material according to any one of claims 1 to 3, wherein the silicon carbide accounts for more than 97% of the mass of the polycrystalline ceramic material.

7. The polycrystalline ceramic material according to any one of claims 1 to 3, wherein more than 90% of the sintered grains have an equivalent diameter of 0.5 to 5 micrometers.

8. The polycrystalline ceramic material according to any one of claims 1 to 3, wherein more than 90% of the sintered grains of the polycrystalline ceramic material are silicon carbide grains in a beta crystalline form.

9. The polycrystalline ceramic material according to any one of claims 1 to 3, wherein the equivalent diameter of the sintered silicon carbide grains in alpha crystalline form is less than 10 micrometers.

10. A polycrystalline ceramic material according to any one of claims 1 to 3, having an electrical resistivity of less than 50 milliohms / cm as measured at 20°C and atmospheric pressure.

11. A method for producing a polycrystalline sintered ceramic material according to any one of claims 1 to 3, comprising the following steps: (a) To produce a supply material containing the following in mass ratio: - A powder of silicon carbide particles having a median size of 0.1 to 5 micrometers, comprising at least 95%, in a beta crystalline form, with a silicon carbide content of at least 95% by mass, and - Less than 3% carbon or carbon precursors with a median diameter of less than 1 micrometer. - Silicon or silicon precursors making up less than 2% of the material, with a median diameter of less than 5 micrometers. (b) Molding the supplied raw material into the shape of a preform, (c) Solid-phase sintering of the preform in a nitrogen atmosphere at a pressure exceeding 60 MPa and at a temperature exceeding 1800°C but less than 2100°C. Here, the supply material contains at least 0.05% of a solid-phase sintering additive containing an element selected from the group consisting of zirconium, titanium, and hafnium.

12. The manufacturing method according to claim 11, wherein the silicon carbide particle powder has a total mass content of less than 3% free or residual carbon, less than 2% free or residual silica, less than 0.5% free or residual silicon, and less than 1% elemental mass content of contaminants or impurities.

13. The manufacturing method according to claim 11, wherein the solid-phase sintering additive is a metal powder, oxide, nitride, carbide, boride, or fluoride of an element selected from the group consisting of zirconium, titanium, and hafnium.

14. The manufacturing method according to claim 11, wherein the supplied raw material does not contain silicon or a silicon precursor and / or does not contain aluminum or an aluminum precursor.

15. An article comprising the material described in any one of claims 1 to 3, wherein the article is selected from the following: an article comprising a turbine, pump, valve or fluid line system; a heat exchanger; a solar heat absorber or device for heat recovery or light reflection; a refractory coating for a furnace; a cooking surface; a crucible for melting metals or metalloids; wear-resistant parts; a cutting tool; a brake pad or disc; a coating or support for thermochemical treatment; or a substrate for active layer deposition for the optical and / or electronics industry; a heating element or resistor; a temperature or pressure sensor; an igniter; or a magnetic susceptor.

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