FR2983850A1 - Preparing crude fiber comprising mixture of ceramic powder and polymer, comprises injecting suspension comprising ceramic powder of identical nature and polymer capable to coagulate in presence of liquid coagulant, in liquid solution - Google Patents

Abstract

Preparing at least one crude fiber comprising a mixture of at least a ceramic powder and at least a polymer, comprises injecting a suspension comprising the ceramic powder of identical nature and the polymer capable to coagulate in the presence of liquid coagulant, in a liquid solution. An independent claim is included for manufacturing at least one ceramic fiber comprising preparing crude fiber, and heat treating the fiber at a temperature and effective duration to decompose the polymer.

Description

TECHNICAL FIELD The present invention relates to the preparation of raw or fired ceramic-based fibers involving the use of powder and a polymer. specific. Such fired fibers, because they are ceramic-based, have a highly refractory character and high mechanical strength and can thus be used as fibrous reinforcements in areas involving high mechanical and thermal stresses. More particularly, they can enter, as fibrous reinforcements, into the constitution of mechanical parts used in the field of aeronautics, civil nuclear, heat transfer. In view of their very wide field of application, ceramic fibers have been the subject of many methods of preparation, as in US Pat. No. 5,135,895, where it is described, in particular, the preparation of ceramic fibers. by extruding a mixture comprising a ceramic powder (s) and a polymer followed by sintering the filaments thus extruded to remove the polymeric portion; US 4,942,011, wherein it is described, in particular, the preparation of ceramic fibers (s) by spinning a mixture comprising a polymeric precursor of the desired ceramics or ceramics followed by sintering the obtained filaments, in order to transform said precursor in said ceramic or ceramics. Regarding the first mode mentioned above, extrusion involves the use of relatively large powders (for example, submicron) and an extrudable polymer in large quantities to constitute a mixture that can be extruded, which, in the end, induces the following drawbacks: the obtaining of fibers having a grain size greater than one micrometer; relatively high costs due to the use of extrudable polymers, which can be expensive; the obtaining of fibers, for which it is difficult to obtain small diameters (such as a diameter ranging from 10 to 15 μm). Regarding the second mode mentioned above, it does not allow the preparation of a wide panel of fibers, insofar as it is dependent on the existence of polymeric precursors of the constitutive ceramic (s) of the fiber. Moreover, these precursors, when they exist, are most often very expensive, which can be prohibitive for the production of fibers in large quantities. Thus, the inventors have set themselves the objective of proposing a process for the preparation of ceramic fibers (s) which are easy to implement and practical and which can be applied to the preparation of fibers based on all types of ceramics. SUMMARY OF THE INVENTION Thus, the invention relates to a method of manufacturing at least one raw fiber comprising a mixture comprising at least one ceramic powder and at least one polymer, said process comprising a step of injecting a liquid suspension comprising: at least one ceramic powder of a nature identical to that intended to enter into the constitution of the aforementioned fiber or fibers; at least one polymer capable of coagulating in the presence of a coagulating liquid solution, in a liquid solution of the aforementioned type, whereby at least one green fiber comprising a mixture of said polymer and said powder results. Before going into more detail in the discussion of this invention, we specify the following definitions. For the purposes of the invention, the term "raw fiber" conventionally means a fiber which has not undergone any heat treatment, which means that, structurally speaking, the fiber has a mixture comprising at least one ceramic powder. and at least one polymer as defined above. By polymer capable of coagulating in the presence of a coagulating liquid solution is meant a polymer capable of gelling in the presence of said solution, this polymer forming at the end of the aforementioned process a mixture, preferably homogeneous with the ceramic powder. For the sake of simplicity, this type of polymer may be called coagulable polymer expression. This process can ideally be used for all types of ceramic powders, provided that they are available (whether commercially or they are prepared beforehand). In particular, this process can be adapted to the preparation of fibers comprising at least one ceramic chosen from carbides, nitrides, borides, silicides, oxycarbides, carbonitrides, oxynitrides, carbosilicures and mixtures of these. this. As carbide fiber (s), mention may be made of titanium carbide (TiC) fibers, boron carbide (B4C) fibers, hafnium carbide (HfC) fibers, silicon carbide fibers ( SiC). As nitride fiber (s), mention may be made of aluminum nitride fibers (A1N) and silicon nitride fibers (Si3N4). As boride fiber (s), mention may be made of titanium boride fibers (TiB2) and hafnium boride fibers (HfB2). As silicide fiber (s), mention may be made of silicide (s) metal fibers, such as titanium silicide (TiSi 2) fibers. As mentioned above, the process of the invention involves an aqueous suspension comprising: a ceramic powder of a nature identical to that intended to enter into the constitution of the aforementioned fiber or fibers; a polymer capable of coagulating in the presence of a coagulating liquid solution. The ceramic powder may be of the same nature as the ceramics already explained above to define the nature of the fibers, that is to say may be a powder made of a ceramic selected from carbides, nitrides, borides, silicides, oxycarbides, carbonitrides, oxynitrides, carbosilicures and mixtures thereof. This powder may have an average size of nanometric or submicron particles, namely an average particle size ranging from 1 to 1000 nanometers, for example, from 40 to 50 nm in diameter. In particular, it may be a carbide powder, and even more particularly, a silicon carbide powder comprising, for example, an average particle size ranging from 20 to 250 nm in diameter. preferably 40 to 50 nm in diameter. However, the powders used may have a larger size, since it is desired to obtain fibers having a larger grain size than a nanometric size of grains. The powder may be used alone or in admixture with other powders, such as powders of a sintering agent and / or a deoxidizing agent.

The sintering agent may be used when the green fibers obtained according to the process of the invention are intended to be heat-treated to remove the polymer, to allow densification of the fibers by reducing their porosity. The deoxidizing agent may be able to prevent and / or eliminate the formation of oxide compounds in the fibers, which could hinder subsequent densification of the latter by sintering. As the powder of a sintering agent, especially when the ceramic powder is silicon carbide, mention may be made of a boron-based ceramic powder (such as B4C, TiB2, AlB2), an aluminum-based powder ( such as Al, AlN), an oxide ceramic powder (such as A1203, Y203, Y3A15012, CaO, MgO) or a silicide ceramic powder (such as MoSi2). As the deoxidizing agent, mention may be made of: carbonaceous inorganic materials, such as: carbon powders, such as graphite powder, diamond powder, carbon black powder, single-walled carbon nanotubes Double-walled carbon nanotubes, multiwall carbon nanotubes; carbon nanocornets of graphene; and * mixtures thereof; organic carbonaceous materials, such as resins, among which mention may be made of phenolic resins and cyanate ester resins. The powder and optionally the powder of a sintering agent and / or deoxidizing agent is (are) present in the solution at a level of at least 2% by volume of the suspension, which does not does not exclude the fact that the raw fibers at the end of the process may contain a significant proportion of powder. The polymer capable of coagulating in the presence of a coagulating liquid solution may be a polymer belonging to the family of polyvinyl alcohols (symbolized PVA), polylactic acids, copolymers of lactic acid and glycolic acid, regenerated celluloses. (as a viscose), alginates and mixtures thereof. Advantageously, the polymer capable of coagulating in the presence of a coagulating liquid solution is a polymer belonging to the family of polyvinyl alcohols. More particularly, it may be a polyvinyl alcohol comprising a repeating unit sequence of formula (I) below: (CH 2 CH OH 25 (I) Preferably, such a polyvinyl alcohol has a molar mass ranging from 30 to 250 kg.morl, for example, a polyvinyl alcohol having a molar mass of 195 kg.mol.

In addition to the presence of at least one powder and at least one polymer as defined above, the suspension may comprise a dispersing agent, namely an agent capable of providing a stable and homogeneous suspension. Without being bound by the theory, the dispersing agent adsorbs itself on the surface of the powder particles, which makes it possible to avoid a phenomenon of sedimentation of the powder and also a phenomenon of coagulation of the aforementioned polymer before bringing it into contact with a specific coagulating liquid solution. It can also be covalently attached to the surface of the powder particles, so that, for example, covalently grafted polyethylene glycols can be left on the surface of the powder. The use of a dispersing agent also makes it possible to obtain stable suspensions having a large volume fraction of powder particles, for example a volume fraction ranging from 1 to 15% and more particularly a volume fraction of about 5%, such that a volume fraction of 50% is obtained in the raw fiber. The dispersing agent is, advantageously, an amphiphilic polymeric compound (i.e., a polymeric compound comprising both polar and nonpolar groups) and preferably having a HLB balance of from 10 to 25, preferably preferably from 30 to 20, for example 15.

Dispersants that effectively respond to these specificities can be: (i) polyol esters; (ii) fatty alcohol ethers and polyethylene glycol ethers; (iii) block copolymers comprising at least one polyethylene block and at least one polyethylene glycol block. Among the polyol esters, mention may be made of: compounds belonging to the family of optionally polyethoxylated fatty acid and sorbitan esters, compounds of this type which may be those sold under the brand name Tween®; Compounds belonging to the family of fatty acid esters and polyethylene glycol. Among the fatty alcohol ethers and polyethylene glycol, there may be mentioned compounds belonging to the family of polyethylene glycols having an alkyl terminal group. As an example of such dispersing agent, polyethylene glycol octadecyl ether having the following formula (II) may be mentioned: This dispersing agent is commercially available from Sigma-Aldrich under the name BriJ S20.

Those skilled in the art will choose, as appropriate, the proportion of dispersing agent to be incorporated in the suspension, so as to obtain a stable and homogeneous suspension, this choice can be made by means of routine tests according to the quantity , the particle size and the nature of the powder or powders used, the quantity of water used and the quantity and nature of the polymer used. Thus, by way of example: when the powder consists of a mixture comprising a silicon carbide powder having an average particle size ranging from 40 to 50 nm, a boron carbide powder having an average particle size of less than 1} gym and a carbon powder having an average particle size of 10 nm in the following proportions: 96%, 1% and 3% for a total mixing mass of 10 g; when the suspension comprises 3 g of PVA polymer with a molar mass of 195 kg.mol; when the suspension comprises 100 g of water; and when the dispersing agent is the compound of formula (II) mentioned above, the dispersing agent may be present in an amount such that its concentration is greater than the critical micelle concentration (for example, of the order of 0, 5 g), whereby: the powder particles are dispersed homogeneously; the suspension does not have aggregates greater than 5 μm in size but preferably has aggregates less than 1 μm in size. Without being bound by the theory, the stability of the suspension, in this case, is ensured by the steric or electrostatic repulsions between the dispersing agent molecules adsorbed on the surface of the powder particles, these repulsions protecting the powder of the particles. attractive interactions of Van der Waals at the origin of powder agglomeration phenomena detrimental to the stability of the suspension. On the other hand, a homogeneous and stable suspension will affect the quality of formation of the green fibers, when brought into contact with the coagulating liquid solution mentioned above. Indeed, when the suspension is highly heterogeneous and sediments rapidly, the presence of the coagulable polymer (such as a polyvinyl alcohol) will lead to the existence of local polymer concentration gradients, which can generate the following phenomena: the polymer-depleted zones induce solidification inequalities of the fiber by coagulation of the polymer; The powder particles are insufficiently bridged together by the polymer; the fiber, in contact with the aforementioned liquid coagulant solution, is fragile and can therefore easily break. As specified above, the suspension is a liquid suspension, which means that it contains an organic solvent, water and / or a mixture thereof. As the organic solvent, there may be mentioned dimethylsulfoxide (known by the abbreviation DMSO), glycerine, ethylene glycol, diethylene glycol, triethylene glycol and mixtures thereof. Preferably, the liquid suspension comprises water, DMSO and / or mixtures thereof. When the liquid suspension comprises water, the pH of the suspension can preferably be maintained at a value of from 3 to 5, this pH being attained by the addition to the dispersion of one or more acids. . The liquid suspension may further comprise boric acid, borates and mixtures thereof. When the slurry comprises water, a person skilled in the art will appropriately choose the proportion of water to be incorporated in the slurry, so as to obtain a stable and homogeneous suspension, this choice being able to be carried out by means of water. routine tests as a function of the amount, particle size and nature of the powder or powders used, the nature and amount of dispersing agent used and the amount and nature of the polymer used. Thus, by way of example: when the powder consists of a mixture comprising a silicon carbide powder having an average particle size ranging from 40 to 50 nm, a boron carbide powder having a lower average particle size to 1} gym and a carbon powder having an average particle size of 10 nm in the following proportions by weight: 96%, 1% and 3% for a total mixing mass of 10 g; when the suspension comprises, as dispersing agent, 3 g of PVA polymer with a molar mass of 195 kg · mol -1 -1; when the dispersing agent is the compound of formula (II) mentioned above at the rate of 5 g; the water may be present in an amount such that the mass ratio between the water and the powder ranges from 6 to 14, whereby: the suspension is homogeneous and does not have aggregates greater than 1 μm in size which can be observed in light microscopy; the suspension has optimum characteristics in order to be brought into contact with a saline solution to form the raw fibers.

For a fixed weight ratio (dispersing agent / powder) and a weight ratio (water / powder), one skilled in the art can also determine, by routine tests, a mass ratio (powder / coagulable polymer), so as to obtain a homogeneous suspension and able to easily turn into raw fibers when placed in contact with a specific coagulating liquid solution. By way of example: when the powder consists of a mixture comprising a silicon carbide powder having an average particle size ranging from 40 to 50 nm, a boron carbide powder having an average particle size of less than 1 and a carbon powder having an average particle size of 10 nm in the following proportions: 96%, 1% and 3% for a total mixing mass of 10 g; when the suspension comprises water in an amount such that the weight ratio water / powder is equal to 8; and when the suspension comprises a dispersing agent of formula (II) mentioned above in an amount such that the mass ratio (dispersing agent / powder) is equal to 0.5, the powder may be present in an amount such that the mass ratio between the powder and the coagulable polymer is equal to a value less than or equal to 3, whereby this suspension in contact with an aqueous saline solution of sodium sulphate (Na 2 SO 4) at 280 μl -1 and brought to 40 ° C. obtaining raw fibers having a good overall strength and may have a length of several tens of centimeters. Prior to the step of injecting the slurry into a coagulating liquid solution, the method may comprise a step of preparing the slurry as such. This preparation step conventionally consists in bringing into contact the ceramic powder (s) and optionally a sintering agent powder and / or a deoxidizing agent powder with a coagulable polymer and water and optionally with a dispersing agent. . By way of example, according to a particular embodiment of the invention, the preparation of this suspension may comprise the following operations: an operation of contacting the powder or powders with a dispersing agent in the presence of water , such as a dispersing agent of formula (II) mentioned above; a mixing operation, for example, under ultrasound, of the mixture obtained in the preceding operation; an operation for adding the coagulable polymer optionally in solution; an agitating operation of the mixture obtained so as to homogenize said suspension. The aforementioned suspension is then intended, in accordance with the process of the invention, to be injected into a coagulating liquid solution, whereby in contact with said solution, the polymer of the suspension coagulates to form at least one raw fiber comprising a mixture comprising the ceramic powder and the coagulable polymer. The coagulation is based on the principle that the solution comprises at least one antisolvent compound of said polymer, allowing the formation of a green fiber in the form of a monofilament or mutifilaments by solidification of said polymer. This can be referred to as wet spinning.

This coagulating liquid solution may comprise a solvent selected from water, a monoalcohol, a polyol and mixtures thereof. Preferably, this coagulating liquid solution comprises a solvent selected from water, methanol, ethanol, butanol, n-propanol, isopropanol, glycol and mixtures thereof. Even more preferably, this liquid coagulant solution comprises a solvent selected from water, methanol, ethanol, a glycol and mixtures thereof. In addition, the coagulating liquid solution, especially when it is an aqueous solution, may comprise one or more salts capable of promoting the coagulation of the coagulable polymer (in which case it may be qualified as a saline solution), such as ammonium salts. (such as ammonium sulfate), alkali salts, such as potassium sulfate, sodium sulfate, sodium carbonate, sodium hydroxide, potassium hydroxide, and mixtures thereof. It may be in particular a saturated solution of salt (s) such (s) defined (s) above. On the other hand, the coagulating liquid solution may comprise one or more additives intended to improve the mechanical properties, the water resistance of the fibers and / or to facilitate the spinning of the fibers, these additives being able to be chosen from boric acid. , borates and mixtures thereof.

The above-mentioned liquid solutions, especially aqueous solutions of sodium sulfate, are particularly suitable for generating coagulation of a polymer belonging to the family of polyvinyl alcohols. The coagulating liquid solution, when it is a saline solution, can advantageously be maintained at a temperature at which the solubility of the salt in the water is maximum, this temperature being able to be determined by routine tests. by those skilled in the art to determine the optimum salt solubilization temperature. The pH of the coagulating liquid solution can advantageously be displaced so as to make it more basic. By way of example, when the saline solution consists of an aqueous solution of sodium sulphate (Na 2 SO 4) at 280 μl -1, the optimum temperature for maintaining the solution can be 40 ° C., at which temperature the solubility of the sulphate sodium in the water is maximum.

From a practical point of view, the suspension can be injected into the saline liquid solution by means of a needle of a diameter suitable for obtaining raw fibers of a desired diameter. The fibers thus obtained in the saline solution may be subjected to a rinsing operation with another solution (whether saline or not) (called washing solution). For example, it may be a saline solution comprising an additive capable of permitting the crosslinking of the polymer used in the constitution of the raw fiber, such as an aqueous solution comprising sodium tetraborate. Such a rinsing operation, when it allows a crosslinking of the polymer used in the constitution of the raw fiber, makes it possible to limit a stretching of the fiber, when it is necessary to isolate the raw fibers from the solution, in which they bathe. The method may comprise, at the end of the injection step and of the optional washing step, a step of isolating the raw fibers. These raw fibers may then be subjected to a washing step, for example with water, so as to remove traces of elements from the coagulating liquid solution and the optional washing solution. The raw fibers thus obtained may be subjected to a heat treatment, so as to decompose the polymer, whereby the fibers thus treated will consist of a porous ceramic when the heat treatment consists only, in a first step, in removing the polymer or consist of a dense ceramic, when the heat treatment is further intended, in a second time, to densify said fiber (for example, by sintering). Thus the invention also relates to a method of manufacturing at least one fiber consisting of a ceramic comprising a step of implementing the method of manufacturing at least one raw fiber as defined above; and a heat treatment step of the raw fiber obtained above at a temperature and duration effective to at least break down said polymer. This heat treatment step may comprise the following operations: an operation of pyrolysis of at least the aforementioned raw fiber at a temperature and duration effective to cause the decomposition of said polymer; a sintering operation of the (the) fiber (s) obtained (s) at the end of the pyrolysis operation at a temperature and duration effective (s) to obtain a cohesion of ceramic grains constituting the heart of By way of example, when the green fiber is a fiber comprising silicon carbide and a polymer belonging to the family of polyvinyl alcohols, the pyrolysis operation can be carried out at a temperature and a duration ranging respectively from 400 to 900 ° C. and from 5 to 60 minutes, for example under argon, while the sintering operation can be carried out at a temperature and a duration which can range, respectively, from 1600 at 2200 ° C. and from 5 to 60 minutes, with a rate of rise in temperature that can range from 5 to 1000 ° C./min, for example, under argon. To avoid decomposition of the constituent ceramic of the fiber during the heat treatment, when it occurs in particular in the solid phase, it may be possible to implement the heat treatment: -on a bed of ceramic powder of the same 5 nature that the one entering the constitution of the fibers, to avoid the losses of ceramic by sublimation (said first case); to reduce the volume of the enclosure, in which the heat treatment takes place, so as to reduce the dilution factor in a rare gas (said second case); to reduce the heat treatment times, in order to limit the sublimation of the constituent elements of the ceramic of the fiber (said third case); and -solving the fibers of the external medium via the formation of a carbon sheath around the fiber (said fourth case); to increase the gas pressure of the heat treatment chamber (said fifth case). As regards the first case, during the heat treatment, the sublimation phenomenon is generated at the level of the constituent powder of the bed and not of the fiber, because of the finer size of the constituent powders of the bed. With regard to the second case, obtaining a reduced heat treatment chamber volume can be achieved by using a specific device, which is in the form of a graphite susceptor 30 heated by induction using a generator capable of delivering a maximum power of 2 kW, the crucible comprising the green fibers being placed in the center of an inert tube, for example made of quartz, traversed by a stream of rare gas (for example, argon ), this crucible can be isolated from the heating system with a carbon felt. Regarding the third case, it may be possible to practice a very high heating rate, for example, a heating rate ranging from 500 to 1000 ° C / min.

Regarding the fourth case, it may be envisaged to practice the deposition of a carbon precursor polymer sheath which materializes, after heat treatment, by the formation of a carbon sheath confining the fiber.

The invention will now be described with reference to the following examples given for illustrative and non-limiting. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS EXAMPLE 2.4 g of a mixture of powders comprising a 13-SiC powder having an average particle size of around 50 nanometers and a specific surface area 30 m 2 / g (96 % mass), a diamond black powder with an average particle size of 10 nm (at 3% by weight) and a B4C powder with an average particle size of less than 1 μm are added to 1 2 g of BriJ S20 dissolved in 20 g of distilled water, the BriJ S20 being a surfactant of the general formula: wherein n is an integer of 20. The mixture is ultrasonically dispersed for 60 minutes at room temperature. using a sonotrode under a power of 200 Watt. To this solution are added 10 g of a solution containing 8% by weight of PVA (Mowiol 56-98, MW = 195 kg-mol-1, degree of hydrolysis of 99%). The mixture is made by magnetic stirring for one hour.

The suspension obtained is injected through a needle 150 micrometers in diameter, with a controlled rate of 10 ml / min, by a syringe pump, into a coagulation bath consisting of a saturated aqueous solution of sodium sulfate (Na 2 SO 4, 280 μL-1) maintained at 40 ° C. The fibers obtained are rinsed in an aqueous solution of sodium tetraborate at 0.05% by weight for one minute and then dried at 75 ° C. The dried fibers are heated at 400 ° C for 10 minutes. The latter are placed in a graphite crucible and heated at 1500 ° C. for 5 minutes under argon. The fibers are then heated to 2000 ° C for 10 minutes under argon following a temperature rise rate of 50 ° C / minute.

Claims (20)

REVENDICATIONS1. A method of manufacturing at least one green fiber comprising a mixture comprising at least one ceramic powder and at least one polymer, said method comprising a step of injecting a suspension comprising: at least one ceramic powder of identical nature to that intended to enter into the constitution of the fiber or fibers mentioned above; at least one polymer capable of coagulating in the presence of a coagulating liquid solution, in a liquid solution of the aforementioned type, whereby at least one green fiber comprising a mixture of said polymer and said powder results. 2. The process according to claim 1, wherein the ceramic powder is selected from carbides, nitrides, borides, silicides, oxycarbides, carbonitrides, oxynitrides, carbosilicides and mixtures thereof. 3. A process according to claim 1 or claim 2, wherein the ceramic powder is a carbide powder. The method of claim 3, wherein the ceramic powder is a silicon carbide powder. The method of any one of the preceding claims, wherein the powder has an average nanoscale particle size. The method of any of the preceding claims, wherein the ceramic powder is used in conjunction with a powder of a sintering agent and / or a deoxidizing agent. 7. Process according to any one of the preceding claims, in which the polymer capable of coagulating in the presence of a coagulating liquid solution is a polymer belonging to the family of polyvinyl alcohols, polylactic acids, copolymers of lactic acid and glycolic acid, regenerated celluloses, alginates and mixtures thereof. 8. A process according to any one of the preceding claims, wherein the coagulant polymer is a polymer belonging to the family of polyvinyl alcohols. 9. A process as claimed in any one of the preceding claims, wherein the coagulable polymer is a polymer comprising a repeating unit sequence of the following formula (I) :( C H2 CH 1 OH (I) 10. A process according to any one of the preceding claims, wherein the suspension further comprises a dispersing agent. 11. The method of claim 10, wherein the dispersing agent is a compound selected from: (i) polyol esters; (ii) fatty alcohol ethers and polyethylene glycol ethers; (iii) block copolymers comprising at least one polyethylene block and at least one polyethylene glycol block. The process according to claim 11, wherein the dispersing agent is a compound belonging to the family of polyethylene glycols having an alkyl terminal group. 13. A process according to any one of claims 10 to 12 wherein the dispersing agent is a polyethylene glycol octadecyl ether compound having the following formula (II): ## STR2 ## 14. A process according to any one of the preceding claims comprising, prior to the injection step, a step of preparing the suspension. 15. A process according to any one of the preceding claims, wherein the coagulating liquid solution comprises a solvent selected from water, a monoalcohol, a polyol and mixtures thereof. 16. A process according to any one of the preceding claims, wherein the coagulating liquid solution comprises a solvent selected from water, methanol, ethanol, a glycol and mixtures thereof. 20 17. A process according to any one of the preceding claims, wherein the coagulating liquid solution is an aqueous solution comprising one or more salts selected from ammonium salts, alkali salts and mixtures thereof. 25 18. The process according to claim 17, wherein the alkaline salt is potassium sulfate, sodium sulfate, sodium carbonate, sodium hydroxide, potassium hydroxide and mixtures thereof . 19. A method of manufacturing at least one ceramic fiber comprising: a step of preparing at least one raw fiber by carrying out the method as defined according to any one of claims 1 to 18; a heat treatment step of said raw fiber at a temperature and time effective to at least break down said polymer. 20. The method of claim 19, wherein the heat treating step comprises: -a pyrolysis operation of at least the aforementioned raw fiber at a temperature and time effective to cause decomposition of said polymer; a sintering operation of the fiber (s) obtained after the pyrolysis operation at a temperature and effective time (s) to obtain a cohesion of the ceramic grains constituting the core of the fiber (s). 25