Method for protecting synthetic graphite from oxidation, and synthetic graphite parts obtained by this method that are resistant to oxidation and hydrothermal conditions

Abstract

The invention relates to a part made from synthetic graphite, preferably isostatic synthetic graphite, impregnated with aluminium metaphosphate Al(PO3)3, and characterised in that the aluminium metaphosphate content is at least 1% by weight for the cubic form and does not exceed 0.4% by weight, and preferably does not exceed 0.3% by weight, for monoclinic forms. Such a part has good resistance to oxidation and can be used under hydrothermal conditions.

Description

[0001] METHOD FOR PROTECTING SYNTHETIC GRAPHITE AGAINST OXIDATION, AND SYNTHETIC GRAPHITE PARTS RESISTANT TO OXIDATION AND HYDROTHERMAL CONDITIONS OBTAINED BY THIS METHOD

[0002] Technical field of the invention

[0003] The invention falls within the field of materials science, and more specifically within the field of graphite. It relates more precisely to isostatic graphite. This synthetic material is suitable for use at high temperatures, but to withstand prolonged contact with high-temperature air, it requires protection against oxidation. The present invention relates to a method for protecting isostatic carbon against oxidation. This method comprises impregnation with an aluminophosphate solution, which penetrates the porosity network of the isostatic graphite to form aluminum metaphosphate with a specific structure. This method also provides improved resistance against exudation under hydrothermal conditions.

[0004] The present invention also relates to isostatic graphite parts obtained by this process, as well as certain new uses of these parts, made possible by their improved resistance to oxidation in atmospheric conditions. It also relates to isostatic graphite parts that exhibit enhanced resistance when exposed to hydrothermal conditions.

[0005] State of the art

[0006] Isostatic graphite is a fine-grained, isotropic synthetic graphite obtained by an isostatic forming process. Typically, the raw material mixture includes coke particles and other graphitic materials, along with binders (e.g., pitch). This paste-like mixture is compacted to form a green piece, which is then subjected to uniform hydrostatic pressure. This compressed green piece is subsequently carbonized (typically at about 1000 °C) to form porous, amorphous carbon. Graphitization occurs through heating at a temperature between approximately 1800 °C and 3000 °C. The resulting graphite piece is dense and homogeneous. It exhibits open, uniform porosity; the pore size and porosity percentage depend on the graphite grade. The average pore diameter is typically between about 1 µm and about 20 µm, and the porosity percentage is typically in the range of 10% to 15%.Isostatic graphite is distinguished from other forms of synthetic graphite, for example extruded graphite, by its isotropic character as well as by the porosity characteristics mentioned above.

[0007] Depending on the nature and size of the graphitic material particles, the nature of the binders and the precise conduct of the manufacturing process, different grades of isostatic graphite can be obtained that adapt to application needs; they are distinguished in particular by their average grain size.

[0008] In general, graphite is thermally very stable and can be used in an inert atmosphere at temperatures approaching 3,000 °C. However, in atmospheric conditions, it begins to oxidize around 350 °C, leading to a progressive mass loss. The rate of mass loss increases with temperature. This oxidation begins at active sites corresponding to defects in the graphite lattice, such as dislocations, stacking defects, atomic vacancies, and the edges of graphitic planes. It is facilitated by the presence of a porous network within the graphite, which leads to oxidative attack not only from the geometric surfaces of the part, but also from within the part in a zone close to the surface.

[0009] This problem is well known in the field of graphite in general, and in the field of carbon fibers. While oxidation does not in itself prevent the use of graphite at high temperatures in contact with air, the designer of graphite parts must take it into account, and there are indeed applications for which graphite cannot be used without protection. This problem is also known in the field of carbon-carbon composites used for manufacturing brake discs, particularly for aircraft, since during braking, under the influence of friction, these discs can be exposed to high temperatures.

[0010] The state of the art reveals several methods for protecting carbon products such as graphite and carbon-carbon composites against oxidation, with the aim of enabling their use at higher temperatures and / or for a longer period.

[0011] Prior art methods generally involve depositing one or more protective layers on the part and / or within its pores. These layers may consist of phosphates, carbides, borides, oxides, or other refractory materials. Insoluble substances are applied in suspension. Multilayer systems with identical or different compositions have been described.

[0012] Single or double layers of SiC or other refractory materials have been described in numerous publications, for example by Yang et al. (“A double layer nanostructure SiC coating for anti-oxidation protection of carbon / carbon composites prepared by chemical vapor reaction and chemical vapor deposition,” published in Ceramics International, 30 (5), July 2013, pp. 5053–5062) and by Cheng et al. (“Fabrication of an SC / Si / MoSi2 multi-coating on graphite materials by a two-step technique,” ​​published in Ceramics International, 38(3), April 2012, pp. 2165–2170). It is known that these layers tend to crack, which limits their effectiveness as a barrier layer.

[0013] Glassy coatings can be deposited: US 3,342,627 (Pure Carbon Company) describes, for example, the protection of synthetic graphite by applying a layer of molten ceramic, glass, or phosphates. US 2006 / 0008647 A1 (Honeywell International) describes the protection of carbon-carbon composites for brake discs by multilayer systems comprising a first layer of phosphate and a second layer of borosilicate glass. US 2021 / 0340072 A1 (Goodrich Corporation) describes the protection of carbon-carbon composites by multilayer glassy phosphate-based systems.

[0014] US 2019 / 0264039 (Safran Landing Systems) describes the protection of carbon-carbon composites used for aircraft brake discs by a first layer comprising aluminum or zinc phosphate, TiB2 powder, and B4C powder, followed by a second layer comprising colloidal silica, TiB2 powder, and borosilicate glass powder. Each of these layers is applied wet and then annealed. In the first layer, the TiB2 powder can be replaced by titanium powder, which, along with the B4C, transforms into TiB2.

[0015] Impregnating carbon with an aqueous medium poses a problem due to carbon's low wettability in water and the high viscosity of phosphate solutions resulting from their high phosphate concentration. The penetration depth remains shallow. For this reason, the aqueous medium contains wetting agents.

[0016] US 5,853,821 (Société Européenne de Propulsion) describes an approach to modifying the wettability characteristics of carbon-carbon composites for aeronautical components by pretreating them with a fluid solution of a wetting agent such as an oxyethylenated fatty acid (or fatty alcohol) or a polyol ester. After drying, this pretreatment improves the penetration of even high-viscosity aqueous aluminum phosphate solutions. Penetration depths of approximately 3 mm to 8 mm can be achieved. US 2010 / 0055466 A1 (Goodrich Corporation) describes the impregnation of carbon-carbon composites with aluminum phosphate solutions containing a multivalent cation and a metallic element such as magnesium.The impregnation of graphite with an aluminophosphate solution was described in documents US 2,685,539 (Great Lakes Carbon Corporation) and EP 2,907,797 A1 (AV EKO-Color); the latter document concerns the impregnation of graphite with aluminum and / or zinc phosphates. EP 0 223 205 A1 (Union Carbide Corporation) describes impregnation with a mixture of chloride and phosphate salts, followed by heat treatment between 500 °C and 600 °C. US 2,906,632 (Union Carbide Corporation) describes impregnation with a solution comprising zinc chloride, aluminum phosphate, and boric acid. During heat treatment, a glassy composition is formed. It has been known since the work of McKee et al. (see in particular the publication “The inhibition of graphite oxidation by phosphorus additives”, published in the journal Carbon, Vol. 22(3), 1984, p.285-290) that phosphates do not modify the mechanism of graphite oxidation, but react with the active sites of graphite where oxidation preferentially takes place; these sites are notably oxygenated sites.

[0017] During its work on the oxidation protection of isostatic graphite, the applicant found that the approaches developed for impregnating carbon-carbon composites are not transferable to isostatic graphite, which generally has a smaller pore size than carbon-carbon composites or other forms of synthetic graphite. In particular, the prior art process for carbon-carbon composites and extruded graphite, which involves impregnating the graphite part with an aluminophosphate solution and then subjecting it to heat treatment, leads to an unsatisfactory result: after a certain period of exposure to air, especially humid air, the isostatic graphite parts treated in this way show traces of exudation of a whitish product on the surface and eventually become covered with a white, viscous layer.It is known that metaphosphates can exist in several crystallographic modifications, and that some phases of metaphosphates are hygroscopic. This exudation renders the treated parts unusable in certain types of environments. For example, this is problematic when considering the use of isostatic graphite parts in air conditioning systems.

[0018] The present invention aims to find a method for protecting isostatic graphite against oxidation effectively and durably, without the drawbacks of the aforementioned prior art. In particular, this method must allow the use of isostatic graphite parts in a hot and humid environment without the appearance of the exudation traces known from the prior art in connection with aluminophosphates. Objects of the invention

[0019] The present invention relates to the protection of synthetic graphite parts against oxidation and / or the influence of hydrothermal conditions. It relates more particularly to synthetic graphite having an average pore size of less than approximately 10 µm. The present invention relates in particular to isostatic graphite, and more specifically to isostatic graphite having an average pore size of less than approximately 10 µm and a pore volume of less than approximately 15%.

[0020] It preferably targets isostatic graphite parts with an average pore size of less than about 8 µm, more preferably less than about 6 µm, and even more preferably less than about 4 µm. The pore volume is less than about 15%, preferably less than about 14%, and even more preferably less than about 13%. It may be less than 12%. A form of synthetic graphite to which the present invention can advantageously be applied is isostatic graphite having an average grain size of less than 3 µm and a pore volume between 10% and 12%. For example, isostatic graphite with an average grain size of less than 2 µm, and preferably less than about 1.6 µm, and a pore volume between about 10.2% and about 11.5% may be used.Even more preferably, isostatic graphite is used, having an average grain size of less than about 1.5 pm and a pore volume between about 10.4% and about 11.4%.

[0021] The inventors found that the approach, known for extruded graphite or carbon-carbon composites, of impregnating the graphite part with an aluminophosphate solution and then subjecting it to heat treatment, does not yield satisfactory results for isostatic graphite. More specifically, a part impregnated with isostatic graphite does not exhibit satisfactory strength under hydrothermal conditions, unlike an extruded graphite or carbon-carbon composite part impregnated and heat-treated under the same conditions.

[0022] According to the invention, this process can be modified to give satisfactory results for isostatic graphite parts exhibiting the pore size characteristics indicated above, namely good, durable protection and the absence of exudation traces. The applicant has found that this difference is related to the course of the reactions that take place during the heat treatment.

[0023] During heat treatment, the reaction proceeds in several stages, as shown below. It involves a polycondensation of aluminum dihydrogen phosphate, during which the hydrated species progressively condense to the water-free metaphosphate Al(POs)3. It is important to note that aluminum metaphosphate can exist in different phases. These include:

[0024] - a cubic phase (space group I43d), comprising rings of four PO4 anions 3'tetrahedral connected by octahedra of aluminium cations;

[0025] - a monoclinic phase (space group P2i / a), comprising rings of six PO4 anions 3 - ;

[0026] - a monoclinic phase (space group le), comprising tetrahedral PO4 anion chains 3 ' ; And

[0027] - a hexagonal phase, comprising rings formed by nine tetrahedral PO4 anions 3 .

[0028] The reaction scheme during an increase in temperature is as follows:

[0029] AI(H2PO4)3 -> AIH2P3O10, 2H2O -> AI(POS)3 monoclinic P2i / a AI(POs)3 cubic

[0030] The P / AI ratio of the aluminum dihydrogen phosphate solution does not change the nature of these phases, but influences the temperature at which they form.

[0031] In the literature (see, for example, G. Tricot et al., "New insights into the thermal evolution of aluminophosphate solutions: A complementary XRD and solid-state NMR study," published in J. Europ. Ceram. Soc. 28(6), pp. 1135–1141; see also G. Tricot et al., "Effect of the P / AI Molar Ratio and Heating Rate on the Composition of Alumino-Phosphate Binders," published in Material 2022, 15, 2337), the cubic Al(POs)3 phase is called "phase A," and the monoclinic Al(POs)3 phase P2i / a is called "phase B." We will use this conventional designation below.

[0032] The applicant found that under hydrothermal conditions, the cubic phase exhibits greater stability than monoclinic phases. It is the cubic phase that forms in extruded graphite and in carbon-carbon composites. Surprisingly, the applicant found that the cubic phase does not form in isostatic graphite, and that the tendency for cubic phase formation depends on the average pore size. In particular, for synthetic graphites with an average pore size of less than approximately 10 µm, the cubic phase does not form during heat treatment.

[0033] Based on this observation, the applicant found a solution for forming the cubic phase within graphite parts with a pore size of less than approximately 10 pm. This solution consists of increasing the P / AI molar ratio of the impregnation solution to at least 3.4 and preferably to at least 3.5.

[0034] A first object of the invention is represented by a method for manufacturing a synthetic graphite part resistant to oxidation in air and to exudation under hydrothermal conditions, in which

[0035] - We supply a part made of synthetic graphite,

[0036] - the synthetic graphite part is impregnated, preferably under pressure, by an Al^PC h impregnation solution having a P / Al molar ratio of at least 3.5, and preferably greater than 3.5;

[0037] - the impregnated part, preferably after drying, is subjected to heat treatment at a temperature between approximately 700 °C and approximately 1000 °C under inert gas, to form cubic-shaped Al(POs)3.

[0038] According to a first variant of this first object, the impregnation solution comprises between 30% and 70% by mass of Al^PC h.

[0039] According to a second variant of this first object, which can be combined with said first variant, said impregnation solution has a P / AI molar ratio between 3.5 and 5.0.

[0040] According to a third variant of this first object, which can be combined with said first variant and / or with said second variant, said graphite part has before impregnation an average pore size of less than 10 pm, preferably less than 8 pm, more preferably less than 6 pm, and even more preferably less than 4 pm.

[0041] According to a fourth variant of this first object, which can be combined with said first variant and / or with said second variant and / or with said third variant, said graphite part has before impregnation a porous volume of less than 15%, preferably less than 14%, more preferably less than 13%, and even more preferably less than 12%.

[0042] According to a fifth variant of this first object, which can be combined with said first variant and / or with said second variant and / or with said third variant and / or with said fourth variant, said graphite part has before impregnation an average pore size of less than 10 pm and a pore volume of less than 15%, preferably an average pore size of less than 10 pm and a pore volume of less than 13%, even more preferably an average pore size of less than 8 pm and a pore volume of less than 12%, and even more preferably an average pore size of less than 6 pm and a pore volume of less than 12%.

[0043] According to a sixth variant of this first object, which can be combined with said first variant and / or with said second variant and / or with said third variant and / or with said fourth variant and / or with said fifth variant, the impregnation followed by the heat treatment is repeated one or more times.

[0044] According to a seventh variant of this first object, which can be combined with said first variant and / or with said second variant and / or with said third variant and / or with said fourth variant and / or with said fifth variant and / or with said sixth variant, said graphite part is impregnated under a pressure of at least 5 bars, preferably under a pressure of at least 7 bars, more preferably under a pressure of at least 8 bars, more preferably under a pressure of at least 9 bars, and even more preferably under a pressure of at least 10 bars.

[0045] According to an eighth variant of this first object, which can be combined with said first variant and / or with said second variant and / or with said third variant and / or with said fourth variant and / or with said fifth variant and / or with said sixth variant and / or with said seventh variant, the impregnation solution is left to act for at least one hour, preferably for at least three hours, more preferably for at least six hours, and even more preferably for at least ten hours,

[0046] According to a ninth variant of this first object, which can be combined with said first variant and / or with said second variant and / or with said third variant and / or with said fourth variant and / or with said fifth variant and / or with said sixth variant and / or with said seventh variant and / or with said eighth variant, said synthetic graphite part is an isostatic graphite part.

[0047] A second object of the invention is represented by a synthetic graphite part, preferably isostatic, impregnated with aluminum metaphosphate Al(POs)3, and characterized in that the aluminum metaphosphate content is at least 1% by mass for the cubic form and does not exceed 0.4% by mass, and preferably does not exceed 0.3% by mass, for the monoclinic forms.

[0048] According to a first variant of this second object, the average pore size of said part is less than 8 pm, preferably less than 6 pm, more preferably less than 5 pm, and even more preferably less than 4 pm. According to a second variant of this second object, which can be combined with said first variant, the pore volume of said part is less than 12%, preferably less than 11%, more preferably less than 10%, and even more preferably less than 9%.

[0049] According to a third variant of this second object, which can be combined with said first variant and / or with said second variant, the total content of aluminium metaphosphate Al(POs)3 is between 1% and 5% by mass, preferably between 2% and 5%, and even more preferably between 2.5% and 5%.

[0050] According to a fourth variant of this second object, which can be combined with said first variant and / or with said second variant and / or with said third variant, the proportion of aluminum metaphosphate Al(POs)3 content in cubic form relative to aluminum metaphosphate Al(POs)3 in monoclinic forms shall be at least 80%, and preferably at least 90%.

[0051] According to a fifth variant of this second object, which can be combined with said first variant and / or with said second variant and / or with said third variant and / or with said fourth variant, said part is selected from the group formed by: bearings, bushings and sealing segments, in particular for use in control valves, in particular for air conditioning systems or motors; rotating shaft seals, in particular for use in compressors; telescopic air piping elements, and bushings or connecting segments between tubes, in particular for use in external linear guides or internal linear guides; high-pressure compressor blade guide elements; seals, and in particular rotating shaft seals, in particular for turbojet engines.

[0052] A third object of the invention is represented by the use of a synthetic graphite part according to the second object or any of its first and / or second and / or third and / or fourth variant and / or fifth variant, or of a part capable of being manufactured by the process according to the first object or any of its first and / or second and / or third and / or fourth variant and / or fifth variant and / or sixth variant and / or seventh and / or eighth variant and / or ninth variant, under at least one of the following conditions:

[0053] (a) at a temperature above approximately 80 °C and a relative humidity above approximately 80%

[0054] (b) at a temperature above 350 °C in air. A fourth object of the invention is represented by the use of a synthetic graphite part according to the second object or any of its first and / or second and / or third and / or fourth variant and / or fifth variant, or of a part capable of being manufactured by the process according to the first object or any of its first and / or second and / or third and / or fourth variant and / or fifth and / or sixth variant and / or seventh variant and / or eighth variant and / or ninth variant, said use being selected from the group formed by:

[0055] - control valves, particularly in bearings, rings or sealing segments, these control valves being able to be used in particular in air conditioning systems or in engines;

[0056] - compressors, particularly for rotating shaft seals, these compressors being able to be used in particular in air conditioning systems;

[0057] - telescopic air piping elements, in particular in external linear guides, internal linear guides, as a ring or segment between tubes; these telescopic piping elements can be used in particular in air conditioning systems or in nacelles;

[0058] - high-pressure compressors, in particular as guide elements for high-pressure compressor blades; these high-pressure compressors can be used in particular in engines;

[0059] - turbojet engines, particularly as rotating shaft seals.

[0060] According to a first variant of this fourth object, the uses mentioned above are preferably carried out in aircraft or aircraft components.

[0061] According to a second variant of this fourth object, which can be combined with said first variant, said synthetic graphite part is an isostatic graphite part.

[0062] Brief description of the figures

[0063] [Fig. 1] refers to Example 4 and shows X-ray diffractograms of an isostatic graphite cylinder after impregnation with an aluminophosphate solution (P / Al molar ratio between 3 and 3.2), taken at different locations along the cylinder (curves a), b), c), d)) to explore the homogeneity of the impregnation and the structure of the aluminophosphate. The peaks marked with a diamond correspond to monoclinic Al(POs)3 (P2α / a); their intensity is indicated by the vertical bars. The main peaks marked with an arrow correspond to graphite.

[0064] [Fig. 2] refers to Example 5 and shows X-ray diffractograms of an isostatic graphite cylinder after impregnation with an aluminophosphate solution (P / Al molar ratio greater than 3.5), taken at two different locations on the cylinder (curves a) (end of the cylinder), b) (center of the cylinder)) to explore the homogeneity of the impregnation and the structure of the aluminophosphate. Peaks marked with a cross correspond to cubic Al(POs)3; their intensity is indicated by the vertical bars. The main peaks marked with an arrow correspond to graphite.

[0065] [Fig. 3] refers to example 8 and shows an NMR spectrum 31 P of a sample of Al(POs)3 with cubic structure before (curve (a)) and after (curve (b)) hydrothermal aging in a climate chamber for 168 h at 95 °C and a relative humidity of 95%, followed by drying at 110 °C for 72 h. No change is seen after aging.

[0066] [Fig. 4] refers to example 8 and shows an NMR spectrum 31 P of a sample of Al(POs)3 with a monoclinic structure (P2? / a), with a small cubic component. The spectrum was recorded before (curve (a)) and after (curve (b)) hydrothermal aging in a climate chamber for 168 h at 95 °C and 95% relative humidity, followed by drying at 110 °C for 72 h. A significant evolution is observed after aging, both chemically and crystallographically.

[0067] [Fig. 5] refers to example 6 and shows the photograph of an isostatic graphite cylinder impregnated according to the prior art (sample 1), after hydrothermal aging.

[0068] [Fig. 6] refers to example 6 and shows a photograph of a CC composite cylinder impregnated according to the prior art (sample 2), after hydrothermal aging.

[0069] [Fig. 7] refers to example 6 and shows a photograph of an extruded graphite cylinder impregnated according to the prior art (sample 3), after hydrothermal aging.

[0070] [Fig.8] refers to example 6 and shows the photograph of another extruded graphite cylinder impregnated according to the prior art (sample 4), after hydrothermal aging.

[0071] [Fig. 9] refers to example 6 and shows the photograph of an isostatic graphite cylinder impregnated according to the invention (sample 5), after hydrothermal aging.

[0072] [Fig. 10] refers to Example 10 and shows a typical curve obtained by mercury intrusion porosimetry on an isostatic graphite sample before impregnation. [Fig. 11] refers to Example 10 and shows a typical curve obtained by mercury intrusion porosimetry on an isostatic graphite sample after impregnation and calcination, according to the invention.

[0073] Detailed description of the invention

[0074] In the context of the present invention, the term "protection against oxidation" refers to the stability of the graphite part in hot air. The temperature is typically above 490 °C. Typical test conditions include, for example, a 500-hour test at a temperature of 480 °C in air; under these conditions, the graphite part according to the invention typically undergoes a mass loss of less than 1.7%, and preferably less than 1.5%.

[0075] The term "hydrothermal conditions" here refers to the stability of the graphite part in hot, humid air. A typical test is carried out at a relative humidity of approximately 80% and a temperature of approximately 80°C.

[0076] We first describe the impregnation process. An aluminophosphate solution is used. In an advantageous embodiment, this is a solution of aluminum dihydrogen phosphate. It can be prepared by dissolving aluminum hydroxide in phosphoric acid and water. These solutions are characterized by their phosphorus-to-aluminum molar ratio (P / AI). The stoichiometric molar ratio of aluminum metaphosphate is 3.0. The solutions used for impregnating extruded graphite and carbon-carbon composites are supplied and used with a slight excess of phosphoric acid to delay precipitation, up to a P / AI ratio of approximately 3.2. The inventors found that the use of such a solution does not lead to a satisfactory result for isostatic graphite: phosphate exudation is observed.

[0077] According to an essential feature of the invention, the aluminum dihydrogen phosphate solution shall have a molar ratio (P / Al) greater than approximately 3.4, and preferably greater than 3.5. This ratio is advantageously between 3.5 and 5; the technical effect of this feature will be described below. The solution may have a mass concentration between 30% and 70%.

[0078] Within the scope of the present invention, the inventors have not found any advantage in substituting aluminum, at least in part, with other metallic elements. Before impregnation, the graphite part is advantageously oven-dried to remove traces of moisture. It is then placed in an autoclave and immersed in the impregnation solution. Impregnation is advantageously carried out under pressure, typically at least 5 bar, preferably at least 10 bar. The impregnation solution is left to act for a certain period, advantageously at least one hour, preferably at least three hours, and even more preferably at least 10 hours.

[0079] The inventors found that under these conditions, the impregnation solution is distributed homogeneously throughout the thickness of a part. With synthetic graphite, preferably isostatic, homogeneous infiltration can be obtained over a thickness of at least 20 mm, preferably at least 40 mm, more preferably at least 60 mm, and even more preferably at least 80 mm. For example, at a pressure between 10 bars and 20 bars, with an aluminophosphate solution of a mass concentration between about 30% and about 70% and a P / AI molar ratio between about 3.4 and about 5, homogeneous infiltration is obtained over a thickness of at least 100 mm with an isostatic synthetic graphite having pores of an average size of less than 10 pm, and even less than 8 pm, 6 pm, 4 pm or 2 pm, and a pore volume of less than 13%, or even less than 12%.

[0080] The amount of solution absorbed by the graphite part is typically between 5% and 10% by mass, preferably between 6% and 9% by mass. After impregnation, the synthetic graphite part can be dried.

[0081] The impregnated part is then subjected to heat treatment. This heat treatment advantageously takes place between approximately 700 °C and approximately 1000 °C, preferably under an inert gas (for example, nitrogen). Aluminum metaphosphate is thus formed, according to the following equation:

[0082] In a crucible and with a P / AI molar ratio of approximately 3 to 3.2, the first stage of this reaction typically occurs from about 300 °C, the second from about 350 °C to about 600 °C; it leads to the monoclinic phase P2i / a (phase B). The cubic phase (phase A) forms at a temperature above about 500 °C. In the context of the present invention, solutions with a higher P / AI molar ratio are used, at least 3.4 and preferably at least 3.5. In a carbon part, it is known that a higher temperature is required to form the cubic phase, namely at least 600 °C to 700 °C. The plaintiff discovered that when a commercially available aluminum aluminophosphate solution with a P / AI ratio between 3 and 3.2 is heated to a temperature above approximately 700 °C, the cubic phase (phase A) is obtained when said solution has been poured into a crucible.Similarly, when the same solution is used to impregnate an extruded graphite block or a carbon-carbon composite block, the cubic phase (phase A) is predominantly obtained. However, under the same conditions, an isostatic graphite block yields the monoclinic P2i / a phase (phase B). The problem is that this phase B is not stable under hydrothermal conditions. It is the reaction product of this phase with water that gives rise to the exudation phenomenon observed only with isostatic graphite.

[0083] Without wishing to be bound by this theory, the inventors believe that under these conditions, the transformation from phase B to phase A does not occur when the pores of the graphite are too small. The solution then consists of increasing the P / AI molar ratio of the impregnation solution to at least 3.4 and preferably to at least 3.5. Under these conditions, heat treatment at a temperature of at least 700 °C leads to phase A (cubic phase). The fraction of monoclinic phases is low. More precisely, in a synthetic graphite part according to the invention, the aluminum metaphosphate content is at least 1 wt% for the cubic form and does not exceed 0.4 wt% for the monoclinic forms. Advantageously, the aluminum metaphosphate Al(POs)3 content in monoclinic form does not exceed 0.2 wt% for the P2i / a form and does not exceed 0.2 wt% for the le form.Preferably, the mass content of aluminum metaphosphate should be at least 1.5%, and even more preferably at least 2.0%, for the cubic phase, and less than 0.2% for the monoclinic phases. Preferably, the proportion of cubic to monoclinic aluminum metaphosphate should be at least 80%, and preferably at least 90%. The total mass content of aluminum metaphosphate Al(POs)3 is between 1% and 5%, preferably between 2% and 5%, and even more preferably between 2.5% and 5%.

[0084] The inventors also found that impregnating solutions with a P / AI ratio of at least 3.5 leads to greater industrial reliability of the impregnation process compared to solutions with a lower P / AI ratio, particularly with large isostatic graphite blocks and considering the aging of the impregnation solution in an industrial environment. The impregnation cycle followed by calcination can be repeated one or more times to increase the amount of phosphate deposited within the porous network; this enhances the protective effect against graphite oxidation.

[0085] Impregnation followed by calcination does not lead to a substantial change in the porosity of synthetic graphite. For synthetic graphite parts according to the invention, a reduction in pore size of approximately 0.1 µm to 0.2 µm, or even 0.3 µm for repeated impregnation-calcination cycles, is typically observed. For example, an unimpregnated isostatic graphite block with an average pore size of 1.2 µm decreases to an average size of 1.1 µm after an impregnation cycle with a P / AI molar ratio of approximately 3.5. The reduction in pore volume depends on the pore size: for an average pore size of approximately 1 µm to approximately 1.5 µm, it is typically on the order of fifteen to twenty percent: for example, a decrease from a value of 10% to a value of 8%.

[0086] The synthetic graphite parts according to the invention exhibit good resistance to oxidation in hot air and good resistance under hydrothermal conditions. Therefore, they can be advantageously used under at least one of the following conditions:

[0087] (a) at a temperature above approximately 80 °C and a relative humidity above approximately 80%

[0088] (b) at a temperature above 350 °C in air.

[0089] They can be used in particular in components for air conditioning systems or in engines.

[0090] For example, they can be used in control valves, particularly in bearings, rings or sealing segments; these control valves can be used in particular in air conditioning systems or in engines.

[0091] They can be used in compressors, particularly for rotating shaft seals; these compressors can be used in particular in air conditioning systems.

[0092] They can be used in telescopic air piping elements, in particular in external linear guides, internal linear guides, as a ring or segment between tubes; these telescopic piping elements can be used in particular in air conditioning systems or in nacelles.

[0093] They can be used in high-pressure compressors, particularly as guide elements for high-pressure compressor blades; these high-pressure compressors can be used in engines. They can also be used in turbojet engines, notably as rotating shaft seals.

[0094] Examples

[0095] Example 1: Characterization of an isostatic graphite block

[0096] Industrial-grade isostatic synthetic graphite was supplied by Mersen. A block measuring 610 mm x 305 mm x 152 mm was cut into 18 slices measuring 30 mm x 305 mm x 152 mm to verify its homogeneity. The slices were numbered 1 to 18, with slice 1 including one face of the block and slice 18 being 25 mm from the opposite face, the remaining 25 mm thickness having been discarded. Each slice was then cut into 12 blanks measuring 74 mm x 48 mm x 30 mm.

[0097] The pore size and total pore volume were determined using mercury intrusion porosimetry, a method known to those skilled in the art. Its principle is based on the fact that the pressure required to force mercury into a pore is inversely proportional to the pore size. The Barrett, Joyner, and Halenda (BJH) adsorption-desorption method was also used.

[0098] Table 1 shows typical results, representing average values ​​across four individual measurements. The block exhibits homogeneous porosity. The ash content is higher in the core of the block than at the periphery, a characteristic typically observed in synthetic graphite blocks and reflecting the slow diffusion of impurities to the block's free surface during graphitization.

[0099] [Table 1]

[0100] Example 2: Preparation or supply of AI(H2PO4)s solutions

[0101] Commercially available aqueous solutions of Al(H2PO4)s were supplied. Their nominal P / Al molar ratio ranged from 3.0 to 3.2, and their concentration ranged from 30% to 50% by mass. Their density was approximately 1.5 g / cm³. 3 The P / AI ratio was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The values ​​shown below are derived from these measurements; for a nominal content between 3.0 and 3.2, slightly different ratios are found, between approximately 2.9 and approximately 3.3.

[0102] Example 3: Technique for impregnating graphite blocks with aluminum metaphosphate solution

[0103] The impregnation was carried out in an autoclave at room temperature. From a graphite blank as described in Example 1, a cylindrical sample 40 mm high and 38 mm in diameter was prepared. This sample was oven-dried for 2 hours at 100 °C to remove moisture from the pores. It was then placed in an autoclave. The autoclave chamber was evacuated (1 mbar) for 2 hours. Next, the aluminum metaphosphate solution was introduced by suction into the pressurized autoclave, and a pressure greater than 5 bar was applied for 16 hours at room temperature. The aluminum metaphosphate solutions were as described in Example 2. The chamber was then de-energized, and the impregnated graphite sample was air-dried for 24 hours. It was determined by weighing that the amount of liquid that had penetrated the graphite block was between approximately 6% and approximately 9% by mass.

[0104] The graphite samples underwent heat treatment in a nitrogen-filled tube furnace at temperatures between 700°C and 1000°C, in one or more stages. The amount of aluminum metaphosphate present in the graphite blocks after this heat treatment was between 1% and 5% by mass. After heat treatment, the white aluminum phosphate crust that had formed on the surface of the sample was removed by sanding with 80-grit sandpaper. This cylindrical sample was then cut into several cylindrical slices with a diameter of 38 mm for characterization and analysis.

[0105] Example 4: Formation of aluminum metaphosphate in different types of carbon

[0106] We supplied three types of samples:

[0107] An isostatic graphite according to example 1, called here "E1";

[0108] A carbon-carbon composite, referred to here as "E2"; An extruded graphite (non-isostatic, with an anisotropic texture), referred to here as "E3".

[0109] We impregnated different samples as described in Example 3, namely: a cylindrical sample prepared from an isostatic graphite blank described in Example 1, a carbon-carbon composite sample, and a sample of extruded graphite of a known type.

[0110] These carbon samples were impregnated with a solution of Al(H2PO4)s and annealed as described in Example 3. All three samples were treated in the same way. After heat treatment at 1000 °C, the samples were cut into cylindrical disks as described in relation to Example 3, and these cylindrical disks were analyzed by X-ray diffraction (copper radiation) to semi-quantitatively determine the phases present. The mass percentage of the phases was calculated using the Rietveld refinement method, well known to those skilled in the art. This method simulates an entire diffractogram, including the intensity, shape, and width of a diffraction peak, to quantify the phases present.

[0111] The results are summarized in Table 2. It should be noted that in samples E1, E2, E3, E4, and E5, the carbon percentage includes the percentage of amorphous phosphate phases, which do not produce a diffraction peak. This amorphous contribution is significant only in samples E4 and E5, both of which are outside the scope of this invention. The AIPO4 phase is a degradation product of aluminum metaphosphate that can form during calcination.

[0112] [Table 2] Sample EO corresponds to a control test for which the solution was poured into a crucible, without carbon. For sample E5, the crystalline phase corresponded to approximately 40% by mass of aluminum metaphosphate, the remainder being amorphous.

[0113] Figure 1 shows a typical diffractogram of sample E1. Curve a) was recorded at one end of the cylindrical sample, curve b) at the center of the cylinder and the center of the cylindrical disk, curve c) at the center of the cylinder and the periphery of the cylindrical disk, and curve d) at a second end of the cylinder opposite the first. The diffractograms are virtually identical throughout the thickness of the cylindrical sample. The marked diffraction peaks correspond to the monoclinic form of the space group P2i / a (phase B). It is noted that under the same heat treatment conditions, the cubic phase (phase A) is obtained from an aluminophosphate solution poured into a crucible (sample EO).

[0114] For comparison, two samples (E4 and E5) were prepared under the same conditions using analogous phosphate solutions in which a portion of the aluminum had been replaced by another cation, such as zinc or barium, with an Al / cation molar ratio between 1 and 5, and a P / (Al + cation) ratio between 3.0 and 3.2. Sample E4 represents isostatic graphite. No crystallized products were observed in the carbonaceous products. During the control test in the crucible (E5), a portion of cubic Al(POs)3 was found, the remainder being a metaphosphate glass.

[0115] Example 5: Protection of isostatic graphite by cubic aluminum metaphosphate (according to the invention)

[0116] A sodium aluminophosphate solution with a mass concentration between 30% and 70% and a P / AI molar ratio greater than 3.5 was prepared from aluminum hydroxide, phosphoric acid, and distilled water, analogously to the procedure described in Example 2. Cylindrical samples of isostatic graphite (diameter 38 mm, height 40 mm) with a pore size less than 10 pm and a pore volume less than 13% were impregnated with this solution, following the procedure described in Example 3. The heat treatment also followed the procedure described in Example 3.

[0117] [Fig. 2] shows the X-ray diffractogram in a slice at the end of the cylinder (curve a)) and at the center of the cylinder (curve b)). The two diffractograms are practically identical. Aluminum aluminophosphate is present in its cubic form.

[0118] Example 6: Behavior of samples under hydrothermal conditions

[0119] We treated six cylindrical samples in a climate chamber, first by aging for 168 hours at a temperature of 95 °C and a relative humidity of 95%, then by drying at 110 °C for 72 hours.

[0120] Table 3 summarizes the properties of the samples and the observations. Figures 5 to 9 show photographs of the surface of the cylindrical samples. Sample 0 was prepared in a crucible and is a powder that is stable under hydrothermal aging conditions.

[0121] [Table 3]

[0122] Sample 5 represents the invention.

[0123] Example 7: Protecting graphite against oxidation

[0124] Ten samples containing monoclinic metaphosphate (phase B) and twelve samples containing cubic metaphosphate (phase A) were supplied. These samples underwent oxidation tests for 500 hours at 480 °C in air to determine the effectiveness of the protection of isostatic graphite against oxidation. The mass loss was determined as a percentage.

[0125] Cubic metaphosphate-protected isostatic graphite (phase A) had an average oxidation rate of 1.17% with a standard deviation of 0.33% (i.e., 1.17 ± 0.33%), while the same monoclinic P2i / a metaphosphate-protected isostatic graphite (phase B) had an average oxidation rate of 1.55% with a standard deviation of 1.11% (i.e., 1.55 ± 1.11%).

[0126] Example 8: Hydrothermal transformation of Al(POs)3

[0127] The phase transformation in two Al(POs)3 samples treated in a climate chamber was studied under the conditions described in Example 6. For this purpose, the NMR spectrum was recorded. 31 P before and after hydrothermal treatment (168 h at 95% RH, followed by drying at 110 °C for 72 h). A Bruker Avance-400 MHz (9.4 Tesla) spectrometer was used, with a triple probe and 3.2 mm diameter rotors, at a rotation speed of 20 kHz. Spectra were recorded with a relaxation time of 300 s and a pulse duration of 2 ps, at a radio frequency of 125 Hz, and under proton decoupling conditions (using a High Power Decoupling (HPDEC) sequence with a decoupling time of 5 ps). Each spectrum resulted from sixteen accumulations (Free Induction Decay - FID).

[0128] Sample A initially exhibited phase A (cubic), while sample B initially exhibited phase B (monoclinic P2i / a). The spectra are shown in [Fig. 3] (sample A) and [Fig. 4] (sample B). Sample A is observed to be stable under hydrothermal conditions, whereas sample B is unstable: a large portion of the Al(POs)3 decomposes into phosphoric acid and various phosphates, and a small portion transforms from phase B to phase A.

[0129] Analysis of the homogeneity of a sample according to the invention

[0130] A cylindrical isostatic graphite bar with a diameter of 38 mm and a length of 300 mm was impregnated with an aluminophosphate solution having a molar ratio of P / Al > 3.5 according to the process described in Example 5. After the described heat treatment, this cylinder was cut into 20 discs with an approximate thickness of 10 mm, numbered successively from P01 to P20. Five discs were analyzed by X-ray diffraction to determine the fraction of the cubic phase of Al(POs)3 (phase A), the AlPO4 content, and the carbon content. The results are summarized in Table 4. [Table 4]

[0131] The good homogeneity of the graphite bar is noticeable.

[0132] Example 10: Characterization of some carbon samples

[0133] The porosity of a typical isostatic graphite block, as supplied for the implementation of the present invention, was measured. The pore volume of an outer slice was (10.91 ± 0.45)%, the pore size was (1.34 ± 0.08) pm, and the ash content was (0.05 ± 0.01)% by mass.

[0134] After impregnation with an aluminum phosphate solution with a molar ratio of P / AI = 3.0, followed by heat treatment at 950 °C, the same grade of isostatic graphite, but at different locations, showed a pore volume ranging from approximately 7.27% to approximately 10.1%, with a pore size ranging from approximately 1.07 µm to approximately 1.32 µm. The ash content after impregnation was approximately 1.75%.

[0135] It should be noted that in the case of unimpregnated graphite the ash content, which is very low, represents the mineral impurities contained in the graphite, whereas after impregnation, the ash content essentially represents the amount of phosphate from the impregnation.

[0136] It is observed that impregnation with the phosphate solution and calcination does not lead to a significant change in porosity.

[0137] For comparison, a typical carbon-carbon composite sample was also characterized: its pore volume was 15.44%, and its average pore size was 16 pm. A typical extruded graphite sample had a pore volume of 18.7% and an average pore size of approximately 16.0 pm. [Fig. 10] and [Fig. 11] show the recording of typical mercury intrusion porosity tests. [Fig. 10] is from a sample of isostatic graphite before impregnation. It has a pore volume of 10.31% and an average pore size of 1.25 pm; the absolute density is 2.1 g / cm³. 3 and the apparent density of 1.88 g / cm³ 3 [Fig. 11] comes from a sample of the same grade of isostatic graphite as that of [Fig. 10], but from a different block. It has a pore volume of 7.96% and an average pore size of 1.25 pm; the absolute density is 1.96 g / cm³. 3 and the apparent density of 1.8 g / cm³ 3The ash content is 1.71% by mass. These two figures were recorded at room temperature, with a relative humidity of approximately 25%, using a Pascal 140 / 240 series instrument and SOLID (Solver of Intrusion Data) software, provided by Thermo Scientific.

[0138] Example 11: Penetration depth of impregnation

[0139] The process according to the invention was used to impregnate parallelepiped-shaped pieces (bars) of isostatic graphite measuring 300 mm x 40 mm x 40 mm with a solution having a P / AI molar ratio slightly greater than 3.5, according to the procedure described in Example 3. After the heat treatment according to the invention, X-ray diffraction analyses were performed on each of the two large surfaces (measuring 300 mm x 40 mm) of the bar, and the bar was then cut at half its thickness to obtain two smaller bars measuring 300 mm x 40 mm x 20 mm. The cut surface of one of these smaller bars was then analyzed by two-dimensional X-ray diffraction. The same proportion of Al(POs)3 was found on all three surfaces thus analyzed, with the same distribution of phases A, B, and C of Al(POs)3 and the same amount of residual AlIPO4.This test shows that under the conditions of the process, penetration into isostatic graphite is total over a thickness of 20 mm, and that the properties after impregnation are homogeneous at the level of a bar with a thickness of 40 mm.

[0140] Example 12: Influence of the P / AI ratio of the impregnation solution

[0141] Samples of isostatic graphite impregnated according to the invention were prepared with two different Al(H2PO4)3 solutions, one with a P / Al ratio of 3 and the other with a P / Al ratio of 4. A heat treatment according to the invention was then carried out. The samples were characterized by X-ray diffraction. Table 5 summarizes the proportion of the different phases observed, expressed as mass percentages.

[0142] [Table 5]

Claims

1. DEMANDS 1. Part made of synthetic graphite, preferably isostatic, impregnated with aluminium metaphosphate Al(POs)3, and characterized in that the aluminium metaphosphate content is at least 1 wt% for the cubic form and does not exceed 0.4 wt%, and preferably does not exceed 0.3 wt%, for the monoclinic forms.

2. Synthetic graphite part according to claim 1, characterized in that its average pore size is less than 8 pm, preferably less than 6 pm, more preferably less than 5 pm, and even more preferably less than 4 pm.

3. Synthetic graphite part according to claim 1 or 2, characterized in that its pore volume is less than 12%, preferably less than 11%, more preferably less than 10%, and even more preferably less than 9%.

4. Synthetic graphite part according to any one of claims 1 to 3, characterized in that the total aluminum metaphosphate Al(POs)3 content is between 1% and 5% by mass, preferably between 2% and 5%, and even more preferably between 2.5% and 5%.

5. Synthetic graphite part according to any one of claims 1 to 4, characterized in that the proportion of cubic aluminum metaphosphate Al(POs)3 relative to monoclinic aluminum metaphosphate Al(POs)3 is at least 80%, and preferably at least 90%.

6. A synthetic graphite part according to any one of claims 1 to 5, characterized in that said part is selected from the group formed by: - bearings, rings and sealing segments, in particular for use in control valves, especially for air conditioning systems or engines; - rotating shaft seals, particularly for use in compressors; - telescopic air piping elements, and rings or segments for connecting tubes, particularly for use in external linear guides or internal linear guides; - the guide elements for the high-pressure compressor blades; - seals, and in particular rotating shaft seals, especially for turbojet engines.

7. A method for manufacturing a synthetic graphite part resistant to oxidation in air and exudation under hydrothermal conditions, in which - a part is supplied with synthetic graphite; - The synthetic graphite part is impregnated, preferably under pressure, with an Al^PC impregnation solution avec un excess phosphoric acid having a molar ratio P / AI of at least 3.5 and preferably greater than 3.5; - the impregnated part, preferably after drying, is subjected to heat treatment at a temperature between approximately 700 °C and approximately 1000 °C under inert gas, to form cubic-shaped Al(POs)3.

8. A process according to claim 7, wherein the impregnation solution comprises between 30% and 70% by mass of Al^PC h.

9. A method according to claim 7 or 8, wherein the impregnation solution has a molar ratio P / AI between 3.5 and 5.

0.

10. A method according to any one of claims 7 to 9, wherein said graphite part is impregnated under a pressure of at least 5 bars, preferably under a pressure of at least 7 bars, more preferably under a pressure of at least 8 bars, more preferably under a pressure of at least 9 bars, and even more preferably under a pressure of at least 10 bars.

11. A method according to any one of claims 7 to 10, wherein the impregnation solution is left to act for at least one hour, preferably for at least 3 hours, more preferably for at least 6 hours, and even more preferably for at least 10 hours, 12. A method according to any one of claims 7 to 11, characterized in that said graphite part has, before impregnation, an average pore size of less than 10 pm, preferably less than 8 pm, more preferably less than 6 pm, and even more preferably less than 4 pm.

13. A method according to any one of claims 7 to 12, characterized in that said graphite part has, before impregnation, a porosity volume of less than 15%, preferably less than 14%, more preferably less than 13%, and even more preferably less than 12%.

14. A method according to any one of claims 7 to 13, characterized in that said graphite part has before impregnation an average pore size of less than 10 pm and a pore volume of less than 15%, preferably an average pore size of less than 10 pm and a pore volume of less than 13%, even more preferably an average pore size of less than 8 pm and a pore volume of less than 12%, and even more preferably an average pore size of less than 6 pm and a pore volume of less than 12%.

15. Use of a synthetic graphite part according to any one of claims 1 to 6, or capable of being manufactured by the process according to any one of claims 7 to 114, under at least one of the following conditions: (a) at a temperature above approximately 80 °C and a relative humidity above approximately 80% (b) at a temperature above 350 °C in air.

16. Use of a synthetic graphite part according to any one of claims 1 to 6 or capable of being manufactured by the process according to any one of claims 6 to 14, said use being selected from the group formed by: - control valves, particularly in bearings, rings or sealing segments, these control valves being able to be used in particular in air conditioning systems or in engines; - compressors, particularly for rotating shaft seals, these compressors being able to be used in particular in air conditioning systems; - Telescopic air piping elements, particularly in external linear guides, internal linear guides, as a ring or segment between tubes; these telescopic piping elements can be used particularly in air conditioning systems or in nacelles; - High-pressure compressors, particularly as guide elements for high-pressure compressor blades; these high-pressure compressors can be used particularly in engines; - turbojet engines, particularly as rotating shaft seals, these uses being preferably made in aircraft or aircraft components.

17. Use according to any one of claims 15 to 16, characterized in that said synthetic graphite part is an isostatic graphite part.

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