EP4330210A1

EP4330210A1 - Dense sintered material of silicon carbide with very low electrical resistivity

Info

Publication number: EP4330210A1
Application number: EP22727955.1A
Authority: EP
Inventors: Giovanni MASSASSO; Costana Bousquet
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2021-04-30
Filing date: 2022-04-29
Publication date: 2024-03-06
Also published as: CN117580814A; FR3122423B3; JP2024515855A; WO2022229577A1; KR20240004649A; FR3122423A3

Abstract

Disclosed is a polycrystalline sintered ceramic material of very low electrical resistivity comprising by mass: - more than 95% silicon carbide (SiC), - less than 1.5% silicon in another form than SiC, - less than 2.5% carbon in another form than SiC, - less than 1% oxygen (O), - less than 0.5% aluminium (Al), - less than 0.5% 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, - between 0.1 and 1.5% nitrogen (N), - the other elements forming the complement to 100%, wherein the grains of the above material have a median equivalent diameter of between 0.5 and 5 micrometres, the mass ratio of SiC alpha (α))/SiC Beta (β) is less than 0.1, and the total porosity represents less than 15% by volume of said material.

Description

DESCRIPTION

Title: DENSE SINTERED MATERIAL OF SILICON CARBIDE WITH VERY LOW ELECTRICAL RESISTIVITY

The invention relates to a dense material based on silicon carbide (SiC) which can in particular be used for its high electrical conductivity properties.

Silicon carbide materials have long been known for their high hardness and high chemical inertness, high thermal and mechanical resistance and high thermal conductivity. This makes them prime candidates for applications such as cutting or machining tool components; turbine components or pump elements subject to high abrasion; pipe valves carrying corrosive products; supports and membranes intended for the filtration or pollution control of gases or liquids; kiln linings and firing supports; heat exchangers and solar absorbers; coatings or substrates for the thermochemical treatment of reactors, in particular for etching, or substrates intended for the electronics industry; heating elements or resistors; sensors for high temperature or pressure or for very aggressive environments; igniters or even magnetic susceptors that are more resistant to oxidation than those made of graphite; or even certain particular applications such as mirrors or other devices in the field of optics. However, this type of material has a variable or even high electrical resistivity (of the order of 0.1 to several tens of ohm.cm at 20° C.) which is limiting in service. In order to reinforce for example the use of this material in particular as an igniter, it was proposed by US3974106, US5045237 or US5085804, various hot sintered silicon carbide materials with additions of aluminium, boron or silicon nitride and/or molybdenum disilicide. However, these materials have a low silicon carbide content or a high porosity, which penalizes their performance, moreover, in particular their thermal conductivity or their properties at high temperature.

More recently the publication “Electrical resistivity of Silicon Carbide ceramics sintered with 1 wt% aluminum nitride and rare earth oxide” in Journal of the European Ceramic Society 32(2012) 4427-4434 by Young-Wook Kim et al. studied the influence of the addition of rare earths associated with AIN on the electrical resistivity of sintered SiC bodies. The starting mixture essentially containing SiC in beta or cubic crystalline form, shaping additives including a siloxane and a phenolic resin, and less than 1% by mass of a rare earth powder and a powder of AlN. This mixture is dried, shaped by unidirectional pressing and then hardened at 200°C in order to obtain a manipulable part which is pretreated at 1450°C before being sintered under a load of 20 Mpa at a temperature of 2050°C under nitrogen. The materials obtained have a relative density greater than 95% and a resistivity between 1.5×10 ^-4 and 2.9. 10 ^-2 ohm.m or between 15 and 290 milliomh.cm depending on the rare earth element added.

As explained by Y.Taki et al in the publication “Electrical and thermal properties of nitrogen doped SiC sintered body” in the journal Japan Society Powder Metallurgy Vol. 65 n°8 2018, the addition of Al in AlN form however leads to a liquid phase favorable to densification but detrimental to the mechanical properties at high temperature. Solid phase sintering techniques without resorting to a liquid phase such as pressureless sintering at from the addition of boron and carbon have also been known for a long time as described for example by US4004934. More recently, as shown in the publication “Pressureless sintering of beta Silicon Carbide nanoparticles” in Journal of the European Ceramic Society 32 (2012) 4393-4400 by A.Malinge et al, the use of silicon carbide starting powders of submicron size of beta crystallographic form made it possible to reach a relative density of the order of 90%. However, the sintering temperature necessary for this densification leads inexorably to the formation of an alpha silicon carbide phase as explained in this publication, which increases the electrical resistivity.

The object of the present invention is therefore to provide a sintered SiC material having a low electrical resistivity, that is to say less than 100, preferably less than 50 milliohm.cm, and high mechanical and thermal properties, including at high temperature.

It has been highlighted by the work of the applicant company, described below, an optimum in terms of physico-chemical composition leading to an extremely low electrical resistivity (less than 50 milliohm.cm at room temperature (20 ° C) while maintaining the lowest possible porosity (less than 10% by volume) without resorting to additives based on aluminum elements or penalizing rare earths at high temperature.This was obtained by an appropriate selection of the starting materials and a particular process making it possible to minimize or even avoid the formation of SiC in the alpha form and any formation of liquid phase at the grain boundaries.It has in fact been found by the applicant company that these two factors could penalize the electrical conductivity. The invention thus relates, according to a first aspect, to a polycrystalline ceramic material consisting of sintered grains with a median equivalent diameter of between 0.5 and 5 micrometers, said material comprising by mass more than 95% of silicon carbide (SiC), preferably more than 97% silicon carbide, and having the following elemental composition by mass:

- less than 1.5% silicon in a form other than SiC,

-less than 2.5% carbon in a form other than SiC, and

-less than 1%, preferably less than 0.75%, preferably less than 0.5% oxygen (O), and -less than 0.5% aluminum (Al) 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% alkaline elements, and -less than 0.5% alkaline earth, and

-preferably less than 0.50% boron (B), even more preferably less than 0.2% boron,

-between 0.05 and 1% nitrogen (N),

- the other elements forming the 100% complement, and in which:

-the mass ratio of the SiC content in alpha crystallographic form (a) to the SiC content in beta crystallographic form (b) of said material is less than 0.1, preferably less than 0.05 and -the total porosity represents less than 15%, preferably less than 12%, more preferably less than 10%, by volume percent of said material.

The elemental composition previously described in the elements Si, C, O, Al etc. means of course in addition to silicon carbide, that is to say in complement of more than 95% (preferably more than 97%) by weight of silicon carbide present in said material. The silicon in a form other than SiC can in particular be present in the form of free silica and/or free silicon (metallic silicon).

The carbon in a form other than SiC can in particular be present in the form of free carbon.

According to other optional but advantageous additional characteristics of said material: the material comprises more than 0.1%, preferably more than 0.5% of silicon in a form other than SiC, in particular in the form of free silica and/or of free silicon (metallic).

- The material comprises more than 0.1%, preferably more than 0.5% of carbon in a form other than SiC, in particular in the form of free carbon.

- The material comprises more than 0.1%, preferably more than 0.5% oxygen (O).

- The material does not include, other than in the form of unavoidable impurities, silicon in a form other than SiC,

- The material does not include, other than in the form of unavoidable impurities, carbon in a form other than SiC. The material does not include, other than as unavoidable impurities, the elements oxygen (O), aluminum (Al), Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,

Dy, Ho, Er, Tm, Yb and Lu, alkalis, alkaline earths The material does not contain, other than as unavoidable impurities, boron.

The material does not include, other than as inevitable impurities of other elements. - the total elemental content of Sodium (Na) + Potassium (K) + Calcium (Ca), cumulatively, is less than 0.5% of the mass of said material.

- the elementary mass content of Aluminum (Al) represents less than 0.3% of the mass of said material. the total elemental content of alkali, alkaline earth, aluminum and rare earth, combined, is less than 2%, preferably less than 1%, more preferably less than 0.5% of the mass of said material.

- the elementary content of Boron (B) is less than 0.2% of the mass of said material, and/or greater than 0.02% of the mass of said material.

- the elemental Zirconium (Zr) content is less than 1.0%, preferably less than 0.8%, preferably less than 0.5% of the mass of said material,

- the elemental Zirconium (Zr) content is greater than 0.02%, preferably greater than 0.05%, preferably greater than 0.1%, preferably greater than 0.2% of the mass of said material.

- the elementary content of Molybdenum (Mo) is less than 0.2% of the mass of said material, preferably less than 0.1% of the mass of said material.

- the elementary titanium (Ti) content is less than 1.0%, preferably less than 0.8%, preferably less than 0.5% of the mass of said material,

- the elementary titanium (Ti) content is greater than 0.02%, preferably greater than 0.05%, preferably greater than 0.1%, preferably greater than 0.2% of the mass of said material.

- the elemental content of Hafnium (Hf) is less than 1.0%, preferably less than 0.8%, preferably less than 0.5% of the mass of said material - the elemental content of Hafnium (Hf) is greater than 0.02%, preferably greater than 0.05%, preferably greater than 0.1%, preferably greater than 0.2% of the mass of said material. - the cumulative elementary content of Zr, Hf and Ti is between 0.05% and 1%.

-the elemental nitrogen content is greater than 0.1%.

- the elemental nitrogen content is less than 0.8%, preferably less than 0.7%, preferably less than 0.5% of the mass of said material.

- the elemental content of iron (Fe) represents less than 0.5% of the mass of said material.

- silicon in a form other than silicon carbide Sic represents less than 1% of the mass of said material. - Carbon in a form other than silicon carbide

Sic represents less than 2% of the mass of said material.

- The mass content of free or residual carbon of said material is less than 1.5%, preferably less than 1.0%. - The mass content of free or residual silica of 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 of said material is less than 0.5%, preferably less than 0.1%.

- Oxygen represents less than 0.4%, preferably less than 0.3%, of the mass of said material.

- Sic represents more than 97%, preferably more than 98% of the mass of said material. the SiC in beta crystallographic form (b) preferably represents more than 90% of the mass of the crystalline phases of said material. - the equivalent diameter of the silicon carbide grains in alpha crystallographic form is less than 10 micrometers.

- By volume, more than 90%, preferably more than 95% of the grains have an equivalent diameter of between 0.5 and 5 micrometers, preferably between 0.5 and 3 micrometers.

- by volume of said material apart from its porosity, more than 90%, preferably more than 93%, more preferably more than 95% of the grains are grains of silicon carbide in beta crystalline form. Grains of silicon carbide in beta crystalline form are understood to mean grains whose mass content of beta SiC is greater than 93%, preferably greater than 95%, preferably greater than 97%.

- the grains of said material, whose equivalent diameter is between 0.5 and 5 micrometers, are essentially in beta crystallographic form.

the grains of silicon carbide in alpha crystalline form represent less than 10%, preferably less than 5% by volume percentage of said material apart from its porosity. According to one embodiment, said material may comprise at least 0.5% of silicon carbide grains in alpha crystalline form, apart from its porosity.

-In particular by volume more than 90%, preferably more than 95%, even more preferably all the grains of silicon carbide in alpha crystalline form have an equivalent diameter of less than 5 micrometers, preferably less than 2 micrometers, or even less than 1 micrometer, the growth of such grains being inhibited according to the invention so as to minimize the electrical resistivity of said material.

- In the material constituting the material according to the invention, the nitrogen is present in the grains by insertion into the crystal lattice of the SiC. Nitrogen is also present at the surface of the constituent grains of the material and at the grain boundaries, as are mainly the elements Si and C.

-The total porosity of said material is less than 5%, preferably less than 4%, more preferably less than 3%, by volume of said material.

the median pore diameter of said material is less than 2 micrometers.

- The material has an electrical resistivity, measured at 20° C. and at atmospheric pressure, of less than 50 milliOhm.cm, preferably less than 30 milliOhm.cm, preferably less than 20 milliOhm.cm.

In this description, unless otherwise specified, all percentages are

-mass for chemical or crystallographic compositions and

-volume for grain or pore sizes.

The invention also relates to a process for manufacturing said material comprising the following steps: a) preparation of a starting charge comprising and preferably consisting essentially of, by mass:

-at least 95% of a powder of silicon carbide particles with a median size of between 0.1 and 5 micrometers, the silicon carbide content of which in beta crystalline form is at least 95% by mass, and - preferably less than 0.2% of a sintering additive preferably comprising boron, and

-less than 3% carbon or a carbon precursor, preferably an uncrystallized or amorphous graphite or carbon powder, the median diameter of which is less than 1 micrometer, less than 2% silicon or a precursor of silicon, preferably a metallic or amorphous, preferably metallic, silicon powder, the median diameter of which is less than 5 micrometers. b) shaping of the starting charge in the form of a preform, preferably by casting, c) solid phase sintering of said preform under a pressure greater than 60 MPa, preferably greater than 75 MPa, or even 80 MPa, and at a temperature above 1800° C. and below 2100° C. under a nitrogenous atmosphere, preferably under dinitrogen.

These possible limited additions of carbon or silicon precursor are intended to react respectively the residual silicon or silica or the residual carbon present in the beta silicon carbide powder in order to form silicon carbide by reaction.

According to other optional and advantageous additional characteristics of said 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 of the silicon carbide powder in beta crystalline form is greater than 5 cm ² /g and/or less than 30 cm ² /g.

- the elementary aluminum content by mass of the silicon carbide powder in beta crystalline form is less than 0.1%.

- the sum of the elementary Na+K+Ca+Mg mass contents of the silicon carbide powder in beta crystalline form is less than 0.2%.

- the sum of the elementary mass contents

Sc+Y+La+Ce+Pr+Nd+Pm+Sm+Eu+Gd+Tb+Dy+Ho+Er+Tm+Yb+Lu of silicon carbide powder in crystalline form beta is less than 0.5 %.

- The mass content of SiC of the silicon carbide powder essentially in beta crystalline form, that is to say whose mass content in beta phase is at least 95%, is greater than 99%. The mass content of free or residual carbon of the silicon carbide powder essentially in beta crystalline form is less than 3%, preferably less than 2%, preferably less than 1.5%.

The mass content of free or residual silica of the silicon carbide powder essentially in beta crystalline form is less than 2%, more preferably less than 1.5%, preferably less than 1%.

The mass content of free or residual silicon of the silicon carbide powder essentially in beta crystalline form is less than 0.5%, more preferably less than 0.1%.

The total elementary mass content of contaminants or impurities, represented by elements or species other than silicon or free silica, residual carbon, of the silicon carbide powder essentially in beta crystalline form is less than 1%.

Said powder of silicon carbide particles has a mass content of free or residual carbon of less than 3%, of free or residual silica of less than 2%, of free or residual silicon of less than 0.5%, and a total elementary mass content of contaminants or impurities less than 1.

The silicon carbide powder essentially in beta crystalline form is bimodal and has two peaks, as measured by laser granulometry, more preferably a first peak whose maximum is between 0.1 and 0.5 micrometers and a second peak whose the maximum is between 1 and 6 micrometers. - The specific surface of the silicon carbide powder essentially in beta crystalline form is between 5 cm ² /g and 30 cm ² /g.

- the starting charge comprises at least 0.05% of a solid phase sintering additive, preferably zirconium and/or titanium and/or hafnium, said additive preferably being a metal powder, oxide, nitride, carbide, boride or fluoride of one of these elements. Preferably, the starting charge comprises less than 1% of a sintering additive comprising the element chosen from zirconium and/or titanium and/or hafnium, said additive preferably being a metallic powder, oxide, nitride, carbide, boride or fluoride of one of these elements. Said powder being of purity greater than 98% by mass, that is to say that the sum of the other elements are present according to a mass content of less than 2%.

- The starting charge does not contain any solid phase sintering additive. The silicon carbide powder can be doped with at least one of the elements zirconium and/or titanium and/or hafnium.

- the starting charge comprises at least 0.05% silicon or silicon precursor.

- The starting charge does not include silicon or silicon precursor.

- the starting charge does not include any deliberate addition of aluminum or aluminum precursor, for example in the form of an aluminum nitride powder or an aluminum powder.

- the starting charge does not include the voluntary addition of rare earths or a precursor of one of the elements among Sc+Y+La+Ce+Pr+Nd+Pm+Sm+Eu+Gd+Tb+Dy+Ho+ Er+Tm+Yb+Lu.

- the starting charge comprises at least 0.05% carbon or carbon precursor. - The starting charge does not include carbon or carbon precursor.

- the median diameter of the sintering powder is less than 2 micrometers, preferably less than 1 micrometer. Preferably, it is a boron carbide powder.

- according to a possible mode, the sintering additive comprises the element zirconium. According to one possible embodiment, the sintering additive is a zirconium carbide, fluoride or boride powder.

- the starting charge comprises at least 0.5%, preferably at least 1%, of a carbon precursor.

- the starting charge comprises less than 0.5% or even no silicon precursor.

- The starting charge may optionally contain less than 1% of organic additives provided that they essentially contain the elements between C, O, H, N, Si. For example, acrylic resins, PEG, siloxanes, vinyl, epoxy, phenolic, polyurethane compounds or resins, derivatives of alkyds or glycerophthalic compounds may be suitable.

Any shaping technique known to those skilled in the art can be applied depending on the size of the part to be produced, provided that all precautions are taken to avoid contamination of the preform. Thus casting in a plaster mold can be adapted by using graphite media between the mold and the preform or oils avoiding too intimate contact and abrasion of the mold by the mixture and ultimately contamination of the preform. These usage precautions mastered by those skilled in the art are also applicable to other stages of the process. Thus, during sintering, the mold or the matrix used containing the preform will preferably be made of graphite. Hot pressing (or “Hot Pressing”), hot isostatic pressing (or “Hot Isostatic Pressing”) or SPS (“Spark Plasma Sintering”) techniques are particularly suitable. Preferably, the sintering under load is carried out by SPS, a sintering process in which the induction heating is carried out by the direct passage of current in a graphite matrix in which the preform is placed. The average temperature rise rate is preferably greater than 10 and less than 100° C./minute. The plateau time at the maximum temperature is preferably greater than 10 minutes. This time may be longer depending on the size of the preform and the load in the oven. The nitrogen used for the sintering atmosphere in step c) has a purity greater than 99.99%, or even greater than 99.999% by volume.

The invention also relates to a device comprising the material according to the invention, said device being chosen from: a turbine, a pump, a valve or a fluid conduit system, a heat exchanger; a solar absorber or a device for recovering heat or reflecting light, a refractory furnace lining, a firing support, a crucible for melting metal or metalloid, a piece of protection against abrasion, a cutting tool, a brake pad or disc, a coating or a support for thermochemical treatment, for example etching, or a substrate for depositing active layers intended for the optics and/or electronics industry; a heating element or resistor; a temperature or pressure sensor; an igniter; a magnetic susceptor. Preferably, the device is chosen from: a turbine, a pump, a valve or a fluid conduit system, a protective part against abrasion, a cutting tool, a brake pad or disc, an element or a heating resistor; a sensor of temperature or pressure; an igniter; a magnetic susceptor; a substrate for depositing active layers intended for the optics and/or electronics industry. Definitions: The following indications and definitions are given, in relation to the preceding description of the present invention:

By polycrystalline material is meant a material having several crystalline orientations or crystals of different crystalline orientation.

-In the ceramic material, the sintered grains together represent the bulk of the mass of said material, the intergranular phase optionally consisting of a ceramic and/or metallic phase or of residual carbon advantageously representing less than 5% of the mass of said material. Unlike so-called liquid-phase sintering, the process for firing the material according to the invention is essentially carried out in the solid phase, that is to say it is sintering in which the additives added allowing sintering or the level of any impurities present do not make it possible to form a liquid phase in an amount such that it is sufficient to allow the rearrangement of the grains and thus bring them into contact with each other. A material obtained by solid phase sintering is commonly referred to as “solid phase sinter”.

By sintering additive, often more simply called

“additive” is understood in the present description to mean a compound usually known to allow and/or accelerate the kinetics of the sintering reaction.

-By impurities we mean the unavoidable constituents, involuntarily and necessarily introduced with the raw materials or resulting from the reactions between the constituents. Impurities are not necessary constituents but only tolerated constituents.

-The elementary chemical contents of the sintered material or of the powders used in the mixture of the manufacturing process of said material are measured according to the techniques well known in the art. In particular, the contents of elements such as for example Al, B, Ti, Zr, Fe, Mo, rare earths, alkalis and alkaline-earths in particular can be measured by X-ray fluorescence, preferably by ICP (“Induction Coupled Plasma” ), according to the contents present in particular by ICP if the contents are less than 0.5%, or even less than 0.2%, in particular according to standard ISO21068-3:2008 on product calcined at 750°C in air up to on weight gain. The mass contents of free silicon, free silica, free carbon and SiC are measured according to standard ISO 21068-2:2008. Those of oxygen and nitrogen are determined by LECO according to ISO21068-3:2008.

The composition in polytypes of SiC and the presence of other phases of the sintered material or of the powders used in the mixture of the manufacturing process of said material are normally obtained by X-ray diffraction and Rietveld analysis. In particular, the respective percentages of alpha and beta SiC phase can be determined using BRUKER's D8 Endeavor equipment using the following configuration:

-Acquisition: d5f80: from 5° to 80° in 2Q, step of 0.01°,

0.34s/step, duration 46min

- Front optic: Primary slit 0.3°; Soller slit 2.5° -Sample holder: Rotation 5 rpm automatic knife -Rear optic: Soller slit: 2.5°; nickle filter

0.0125mm; PSD: 4°. 1D detector (Current values).

The diffractograms can be analyzed qualitatively with the EVA software and the ICDD2016 database, then quantitatively with HighScore Plus software according to a Rietveld refinement.

The volume percentages of grains of the sintered material in alpha or beta form and their diameter can be determined by analyzing images from observations by electron backscatter diffraction EBSD (or "electron electron backscatter diffraction"). The installation can be for example composed of a scanning electron microscope (SEM) equipped with an EBSD detector and a spectrometry with energy dispersive X-ray spectrometry (EDX).The EBSD and EDX detectors are controlled by the ESPRIT software ( version 2.1). Images of high crystallographic contrast and/or high density contrast can be collected using available software. The equivalent diameter of a grain corresponds to the diameter of the disk with the same surface as that of the said grain observed according to a cutting plane of the material By observing different sections of material along at least two perpendicular planes, it is possible to have a very good representation of the volume distribution of the different equivalent diameters of the grains and to deduce therefrom the median equivalent diameter (or percentile D50) of said grains by volume. apart from its porosity.

This median diameter (or percentile D50) of grains corresponds to the diameter dividing the grains into first and second equal populations, these first and second populations comprising only grains having an equivalent diameter greater, or less, respectively, than the median diameter. Using the same method as previously described, it is also possible to calculate the volume of any intergranular phases present. The total porosity (or total volume of pores) of the material according to the invention corresponds to the total sum of the volume of closed and open pores divided by the volume of the material. It is calculated according to the ratio expressed as a percentage of the apparent density measured according to IS018754 on the absolute density measured according to ISO5018.

The median particle diameter (or the median "size") of the particles constituting a powder can be obtained by characterization of the particle size distribution, in particular by means of a laser particle sizer. The characterization of the particle size distribution is conventionally carried out with a laser particle sizer in accordance with the ISO 13320-1 standard. The laser particle sizer can be, for example, a Partica LA-950 from the company HORIBA.

Within the meaning of the present description and unless otherwise stated, the median diameter of the particles designates respectively the diameter of the particles below which there is 50% by mass of the population. set of particles, in particular of a powder, the percentile Dso, i.e. the size dividing the particles into first and second populations equal in volume, these first and second populations comprising only particles having a larger size , or less respectively, than the median size.

The specific surface is measured by the B.E.T. (Brunauer Emmet Teller), described for example in the Journal of American Chemical Society 60 (1938), pages 309 to 316.

A powder of silicon carbide particles in beta crystalline form is understood to mean a powder for which the 3C or cubic crystallographic form represents more than 95% by mass of silicon carbide. The alpha crystallographic forms of SiC being mainly the hexagonal or rhombohedral phases: 3H; 4H; 6H and 15R. Unless otherwise indicated, in the present description, all the percentages are mass percentages.

Figures: Figure 1 is an image taken under a scanning microscope of a polished section of the sintered material of example 3 according to the invention.

Examples of embodiments A non-limiting example allowing the production of a material according to the invention is given below, obviously also non-limiting of the methods allowing such a material to be obtained and of the method according to the present invention.

Comparative examples demonstrating the advantages of the present invention are also given below.

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 initially made by casting a slip into a plaster mold according to different formulations given in Table 1 below. afterwards from the following raw materials:

1°) a powder of Sic silicon carbide particles in beta crystallographic form, which has a bimodal distribution with a first peak whose maximum is located at 0.3 micrometers and a second peak of height substantially twice as high as the first and whose maximum is located at 3 micrometers, according to a non-cumulative size distribution measured by number laser granulometer. The median diameter of the bimodal powder is 1.5 µm. This Sic powder has the following elementary mass contents:

-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; H f <0.1

Its mass contents of free Carbon, Silica and Silicon are respectively less than 2.0%, 1.0% and 0.1%. Its mass content of beta SiC phase is greater than 95%. 2°) a powder of carbon black (carbon black) supplied by

Timcal under grade C65 with BET specific surface area of 62 m ² /g. 3°) a boron carbide powder supplied by HC Starck under grade HD-15 with a median diameter of 0.8 μm.

4°) a zirconia powder supplied by Saint-Gobain Zirpro under the grade CY3Z-RA with a median diameter of 0.3 μm.

5°) a titanium oxide powder supplied by Sigma-Aldrich under the grade with a median diameter of 0.1 μm.

6°) an aluminum nitride powder supplied by Nanografi under the grade with a median diameter of 0.06 μm. Pellets thus produced are dried at 50° C. in air. The pellets of Examples 1 and 2 (comparative) are sintered in an oven under Argon at a temperature of 2150° C. for 2 hours without pressure or load. The pellets of Example 3 (according to the invention) and of Example 5 (comparative) are loaded into equipment to carry out SPS-type sintering at 2000° C. under a load of 85 MPa (megapascals) under atmosphere of nitrogen. Unlike Example 3, the sintering of the pellets of Example 4 (comparative) is carried out under vacuum. Example 6 according to the invention is carried out under the same conditions as example 5, but the boron carbide powder is replaced by a zirconia powder, just as in example 8 (also according to the invention) difference from Example 6, in Example 7 (according to the invention) the addition is made in the form of a titanium oxide powder. In Examples 9 and 10 (comparative) unlike Example 7, the addition consists of an aluminum nitride powder. The sintering of examples 9 and 10 is respectively the same as that of example 7 (sintering under load and under N2) and that of Example 4 (sintering under load and under vacuum).

The total porosity of the material obtained is calculated by taking the difference between 100 and the ratio expressed as a percentage of the apparent density measured according to IS018754 on the absolute density measured according to ISO5018. The free silica (SiCt) content is measured by HF attack. Free carbon, oxygen and nitrogen contents are measured by LECO. The free silicon content is measured by aqua regia attack, followed by titration. The other elementary contents are measured by X-ray fluorescence and ICP. The percentage of Sic in beta form and the b/a crystallographic form ratio of SiC are determined by X-ray diffraction analysis according to the method described previously.

The electrical resistivity is measured at ambient temperature (20° C.) according to the Van der Pauw method at 4 points on a sample with a diameter of 20-30 mm and a thickness of 2.5 mm. The volume percentages of grains of the sintered material in alpha or beta form and their diameter were determined by analyzing images from EBSD observations. The installation is composed of a scanning electron microscope (SEM) equipped with a Bruker e-FlashHR+ EBSD detector equipped with the FSE/BSE Argus imaging system and a Bruker XFlash® 4010 EDX detector with an active surface of 10 mm ² . The EBSD detector is mounted on one of the rear ports of the FEI Nova NanoSEM 230 field emission gun SEM with a tilt angle of 10.6° from the horizontal in order to increase both the EBSD signal and the EDX signal. Under these conditions, the optimum working distance WD (ie, distance between the pole piece of the SEM and the analyzed zone of the sample) is approximately 13 mm. The EBSD and EDS detectors are controlled by ESPRIT software (version 2.1). FSE (high contrast) images crystallographic) and/or BSE (high density contrast) were collected using the Argus system by positioning the EBSD camera at a DD distance (sample detector distance) of 23 mm in order to be less sensitive to the topography of the sample. EBSD measurements were performed in point and/or mapping mode. For this, the EBSD camera was positioned at a DD distance of 17 mm in order to increase the signal collected.

The equivalent diameter of a grain corresponds to the diameter of the disk with the same surface as that of said grain observed according to a cutting plane of the material. By observing different sections of material along at least two perpendicular planes, it was possible to determine the distribution of the different equivalent diameters of the grains in the volume of the material and to deduce therefrom the median equivalent diameter of the said grains in volume.

The characteristics and properties obtained according to Examples 1 to 6 are given in Table 1 below. Table 1:

NM = Not measured

The comparison of examples 3 and 6 to 8 according to the invention with the other comparative examples shows that it is possible to obtain, according to the precise and unique conditions of the invention, a crystallized silicon carbide material with low porosity and low even very little electrically resistive, i.e. starting from a pure mixture of SiC in beta form, a very small quantity or even no sintering additive and/or carbon and sintering under pressure in the presence of a nitrogenous atmosphere. Examples 9 and 10 show that the addition of aluminum leads to a much higher resistivity regardless of the sintering mode.

Claims

1. Polycrystalline ceramic material consisting of sintered grains with a median equivalent diameter of between 0.5 and 5 micrometers, said material comprising by mass more than 95% of silicon carbide (SiC) and having the following elementary composition, by mass:

- less than 1.5% silicon in a form other than SiC,

- less than 2.5% carbon in a form other than SiC, - less than 1.0% oxygen (O),

- less than 0.5% aluminum (Al)

- 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,

- less than 0.5% alkaline elements,

- less than 0.5% alkaline earth metal,

-between 0.05 and 1% nitrogen (N),

- the other elements forming the 100% complement, in which:

- the mass ratio of the SiC content in alpha crystallographic form (a) to the SiC content in beta crystallographic form (b) of said material is less than 0.1,

the total porosity represents less than 15%, in volume percentage of said material.

2. Polycrystalline ceramic material according to the preceding claim, having the following elementary composition, by mass:

-less than 0.5% oxygen (O) and/or -less than 0.2% boron (B).

3. Polycrystalline ceramic material according to the preceding claim, in which the total elemental content of Sodium (Na) + Potassium (K) + Calcium (Ca), cumulatively, is less than 0.5% of the mass of said material.

4. Polycrystalline ceramic material according to one of the preceding claims, wherein the elemental nitrogen content is less than 0.5% of the mass of said material.

5. Polycrystalline ceramic material according to one of the preceding claims, in which the elemental content of iron (Fe) represents less than 0.5% of the mass of said material.

6. Polycrystalline ceramic material according to one of the preceding claims, wherein the elemental content of an element selected from the group consisting of zirconium, titanium, hafnium is greater than 0.02% and less than 1%.

7. Polycrystalline ceramic material according to one of the preceding claims, in which the cumulative elemental content of Zr, Hf and Ti is between 0.05% and 1%.

8. Polycrystalline ceramic material according to one of the preceding claims, in which the SiC represents more than 97%, preferably more than 98% of the mass of said material.

9. Polycrystalline ceramic material according to one of the preceding claims, wherein by volume of said material apart from its porosity, more than 90% of the grains have an equivalent diameter of between 0.5 and 5 micrometers.

10. Polycrystalline ceramic material according to one of the preceding claims, wherein by volume of said material apart from its porosity, more than 90% of the grains of said material are grains of silicon carbide in beta crystalline form.

11. Polycrystalline ceramic material according to one of the preceding claims, in which the diameter equivalent of silicon carbide grains in alpha crystallographic form is less than 10 micrometers.

12. Polycrystalline ceramic material according to one of the preceding claims having an electrical resistivity, measured at 20° C. and at atmospheric pressure, of less than 50 milliOhm.cm.

13. Process for manufacturing the polycrystalline sintered ceramic material according to one of the preceding claims, comprising the following steps: a. preparation of a starting charge comprising by mass:

-at least 95% of a powder of silicon carbide particles with a median size of between 0.1 and 5 micrometers, the silicon carbide content of which in beta crystalline form is at least 95% by mass, and - preferably less than 0.2% of a solid phase sintering additive, said additive advantageously comprising boron, and

- less than 3% carbon or a carbon precursor, the median diameter of which is less than 1 micrometer, - less than 2% silicon or a silicon precursor, the median diameter of which is less than 5 micrometers. b. shaping of the starting charge in the form of a preform, preferably by casting. vs. solid phase sintering of said preform under a pressure greater than 60 MPa and at a temperature greater than 1800° C. and less than 2100° C. in a nitrogenous atmosphere.

14. Manufacturing process according to the preceding claim, in which said powder of silicon carbide particles has a mass content of free or residual carbon of less than 3%, of free or residual silica of less than 2%, of free or residual silicon of less than 0.5%, and a total elementary mass content of contaminants or impurities of less than 1%.

15. Manufacturing process according to the preceding claim, in which the starting charge comprises less than 0.2% of a solid-phase sintering additive preferably comprising boron.

16. Manufacturing process according to one of claims 13 to 15, in which the starting charge comprises at least 0.05% of a solid-phase sintering additive, preferably zirconium and/or titanium and/or hafnium, said additive preferably being a metal, oxide, nitride, carbide, boride or fluoride powder of one of these elements.

17. Manufacturing process according to one of claims 13 to 16, in which the starting charge does not comprise does not comprise silicon or a silicon precursor, and/or does not comprise aluminum or an aluminum precursor.

18. Device comprising the material according to one of claims 1 to 12, said device being chosen from: a turbine, a pump, a valve or a fluid conduit system, a heat exchanger; a solar absorber or a device for recovering heat or reflecting light, a refractory furnace lining, a firing support, a crucible for melting metal or metalloid, a piece of protection against abrasion, a cutting tool, a brake pad or disc, a thermochemical treatment coating or support, a substrate for depositing active layers intended for the optics and/or electronics industry; a heating element or resistor; a temperature or pressure sensor; an igniter; a magnetic susceptor.