FR2873687A1 - Baked porous ceramic mousse with a total porosity of between 50 and 92% and an intergranular porosity of at least 5% and means of fabricating such a mousse - Google Patents

Abstract

A mixture M containing a ceramic powder in suspension, at least one gelling agent and at least one foaming agent is prepared at a temperature above the gelling temperature of the gelling agent. The mixture M contains silicon carbide and the baking is carried out in conditions allowing the silicon carbide to consolidate by an evaporation-recrystallisation mechanism : The mousse contains recrystallised silicon carbide, preferably at least 95% by mass, and an intergranular porosity of at least 8%, preferably 10%. The intergranular porosity is formed by pores with a size less than 10mu m, on average 10 to 100 times smaller than the cellular pores. The mean size of the cellular pores is between 80 and 120 mu m and/or the mean size of the intergranular pores is between 1 and 10mu m. More than 15% of the cellular pores have a size greater than 300mu m and/or at least 10% of the cellular pores have a size smaller than 10mu m and/or 20 to 85% of the cellular pores have a size smaller than 300mu m and/or 4 to 40% of the cellular pores have a size greater than 450mu m, the percentages being percentages of number. An independent claim is made for a method of making such a porous ceramic mousse, where a mixture M containing a ceramic powder in suspension, at least one gelling agent and at least one foaming agent is prepared at a temperature above the gelling temperature of the gelling agent. The mixture is brought to a foaming temperature above the gelling temperature to obtain a mousse. The mousse is gelled by cooling the mixture to below the gelling temperature, then dried to obtain a preform and the preform is baked to obtain a porous ceramic mousse. The size of the particles of ceramic powder used in the mixture are diestibuted according to a bimodal distribution. The mixture M has a stabilising agent added to it where the instantaneous viscosity, in Pa.s, increases by at least a factor of ten when the shearing speed of the stabilising agent drops from 100 s->1 to 0 s->1. The stablising agent is xanthane, carraghenane or a mixture of these. A plastifier is added to the mixture M in a quantity by weight of between 0.25 and 1 times that of the gelling agent.

Description

The invention relates to a fired ceramic foam having a

  intergranular porosity, in particular a recrystallized silicon carbide foam, to a process for obtaining such a foam and to uses of a

such foam.

  Porous ceramic foams are known porous products characterized by low density (5 to 50% of theoretical density). They can be made from the vast majority of ceramic powders, in particular alumina, silicon carbide.

  There are several manufacturing processes for obtaining porous ceramic foams.

  The simplest of these methods involves the addition of a blowing agent in a ceramic slurry or press mixture. The whole is shaped then undergoes a cooking cycle which allows to burn the porogenic agent. While burning, the porogenic agent gives way to porosity. This process is described, for example, in patent EP 1 140 731 B1. It has the disadvantage of leading to a gaseous release (possibly toxic) due to the pyrolysis of the pore-forming agent. Moreover, it is difficult to manufacture thick parts, because the gas evolution is then difficult to perform (must be able to evacuate the gas homogeneously).

  Another technique is the replication of a polymer foam, polyurethane for example. This foam is covered with a ceramic slip. The whole then undergoes a cooking cycle that burns the polyurethane foam and sinter the ceramic foam whose structure is induced by the starting polyurethane foam. This process is described for example in US Pat. No. 4,024,212. The disadvantages of gas evolution are found (on burning, the polyurethane releases C-N bonded gases which are dangerous). Moreover, it is difficult to make parts with pores smaller than 200pm.

  An alternative technique is the creation of porosities by the formation of gases by chemical reaction between different additives of a ceramic slip. This process is described, for example, in US Pat. No. 5,643,512 or US Pat. No. 5,705,448. Again, we find the disadvantages associated with the release of gas. In addition, it is necessary to associate with this process a technique of consolidation of the foam.

  One technique is to mechanically introduce a gas into a slurry by stirring. Again, this process must be combined with a technique of consolidation of the foam obtained.

  Among the foam consolidation techniques that can be used for the implementation of the last two techniques mentioned above, there are also several possibilities.

  A first solution consists in mixing polymerizable monomers with the ceramic slip. Under particular conditions, the polymerization of the monomers causes the consolidation of the foam. This process is described for example in patent EP 759 020 B1. On the other hand, the high cost of the monomers that can be used as well as the difficulty of controlling the polymerization conditions have prevented this technique from developing industrially.

  A second solution is to crosslink polymers in a ceramic slip. As for the previous technique, this crosslinking causes consolidation of the foam. This process is described for example in patent EP 330 963. On the other hand, the high cost of crosslinking agents that can be used as well as the difficulty of controlling the crosslinking conditions have prevented this technique from developing industrially.

  A more recent process for producing porous ceramics is described in patent application EP 1 329 439 A1. This process comprises the following steps: 1) Preparation of a suspension of a ceramic powder in a dispersant, 2) Preparation of a solution containing a biogel, also called hydrocolloid, and keeping at a temperature above the gelling temperature of the solution, 3) mixing said suspension and said solution, with the addition of a foaming agent, until a foam, the temperature being kept high enough to avoid gelling the biogel, 4) Pouring the foam into a mold, 5) cooling until gelation of the biogel, 6) drying and baking of the foam obtained.

  The use of a biogel to consolidate the foam avoids the toxicity problems mentioned above. However, it turns out that if the foam cast in the mold has a thickness greater than 60 mm, the foam does not have a homogeneous structure. In addition, according to this method, the amount of water in the final mixture is large (45 to 50% by weight) which causes drying difficulties, especially for large parts. It is therefore not possible to manufacture porous ceramic parts of homogeneous structure and having such dimensions.

  The French patent application filed by the Saint-Gobain European Research and Studies Center Company under the number FR 0408330 describes an improved process making it possible to manufacture porous ceramic foam parts of homogeneous density having dimensions greater than or equal to 60 mm.

  For this purpose, FR 0408330 proposes a method of manufacturing a porous ceramic part comprising the following successive steps: a) preparation of a mixture M containing a ceramic powder in suspension, at least one gelling agent and at least one foaming agent, at a mixing temperature higher than the gelling temperature of said gelling agent, b) shearing said mixture M at a foaming temperature higher than said gelling temperature, until a foam is obtained, c) gelling said foam by cooling said mixture M at a temperature below the gelation temperature of said gelling agent, d) drying said gelled foam so as to obtain a preform, e) curing by high temperature treatment of said preform so as to obtain a fired ceramic foam.

  According to this method, a stabilizing agent is added to said mixture M whose instantaneous viscosity, in Pa.s, increases by at least a factor of ten when a shear rate of said stabilizing agent decreases from 100 s to 0 s -1.

  The inventors of the invention described in FR 0408330 have observed that the slump of the foam during the implementation of the method according to EP 1 329 439 A1 occurs during a critical period extending between the end of the step of shearing and the beginning of gelation. During this period, the gelling agent does not contribute substantially to the structural stabilization of the foam which, for thicknesses of more than 60 mm, collapses under its own weight. Having managed to identify the cause of the slump, they proposed to add a stabilizer in the mixture. The stabilizing agent is chosen for its ability to considerably increase the viscosity of the mixture as soon as the shear of the mixture ceases, which makes it possible to sufficiently stiffen the foam to prevent it from collapsing until the gelling agent gels and can exert its stabilization function. It thus becomes possible to manufacture porous ceramic foam parts of homogeneous density having dimensions greater than or equal to 60 mm and / or having complex shapes (cone or hollow cylinder, hollow sphere portion, etc.).

  The choice of the ceramic powders used in the various processes described above is generally guided by the stresses to which the foams in service are subjected, in particular by the constraints of mechanical strength or thermal conductivity.

  In certain applications, for example for use as a catalyst support, one also seeks the highest possible available surfaces.

  There is therefore a permanent need for ceramic foams with ever higher available surfaces or, with equivalent available surface, a lower density.

  The object of the present invention is to satisfy this need.

  According to the invention, this object is achieved by means of a porous ceramic fired foam having a total porosity of between 50 and 92% (ie a density of between 8 and 50% of the theoretical density). of the material constituting said foam) and an intergranular porosity of at least 5%, preferably at least 8%, more preferably at least 10%.

  Unlike ceramic foams according to the state of the art, whose cell walls are dense, the ceramic foam according to the invention therefore has an intergranular porosity. As will be seen in more detail in the following description, this intergranular porosity develops the available surface on the cell walls of the foam. At identical cell densities, a foam according to the present invention thus has a surface exposed to the much larger external environment. The foam according to the invention thus represents a considerable technical progress in the search for porous ceramic foams having a high available surface.

  Preferably, the ceramic fired foam with porous walls according to the invention, or with intergranular porosity, still has one or more of the following characteristics: the intergranular porosity is formed by pores with a size of less than 10 μm, preferably less than 10 μm; at 8 pm, preferably on average about 5 pm.

  The size of the intergranular pores is on average 10 to 100 times smaller than that of the cell pores. Preferably, the average cell pore size is between 80 and 120 μm, preferably around 100 μm, and / or the average size of the intergranular pores is between 1 and 10 μm, preferably around 5 μm.

  Preferably, at least a portion of the cells, preferably all the cells, are delimited by walls at least partly, preferably completely, of recrystallized silicon carbide. More preferably, the porous ceramic foam with intergranular porosity comprises at least 95% by weight of recrystallized silicon carbide. Preferably, the porous ceramic foam with intergranular porosity comprises more than 98% by weight, more preferably substantially 100% of recrystallized silicon carbide.

  - More than 15% by number of cell pores have a size greater than 300pm.

  Less than 10% by number of cell pores are less than 10 μm in size, and / or 20 to 85% of cell pores are less than 300 μm in size and / or 4 to 40% of cell pores are greater than 450 μm in size. percentages being percentages by number.

  The porous ceramic foam has a surface area of less than 1500, preferably less than 1000, more preferably less than 700, cellular pores per dm2.

  - The porous ceramic foam has a thickness greater than 60 mm.

  The invention also relates to a method of manufacturing a porous ceramic foam according to the invention comprising the following successive steps: a) preparation of a mixture M containing a ceramic powder in suspension, at least one gelling agent and at least one agent foaming agent, at a mixing temperature higher than the gelling temperature of said gelling agent, b) shearing said mixture M at a foaming temperature above said gelation temperature, until a foam is obtained, c) gelling said foam by cooling said mixture M to a temperature below the gelation temperature of said gelling agent, d) drying said gelled foam so as to obtain a preform, e) baking said preform so as to obtain a porous ceramic foam, remarkable in that that the mixture M comprises silicon carbide and that the cooking in step e) is carried out in ns conditions allowing consolidation by evaporation-recrystallization mechanism of said silicon carbide.

  In the remainder of the description, this cooking is called high temperature cooking. This cooking leads to the formation of recrystallized silicon carbide.

  Preferably, the process according to the invention also has one or more of the following characteristics: Preferably, the ceramic powder used in step a) has more than 95% by weight of silicon carbide.

  - The particle sizes of the ceramic powder implemented in step a) are distributed in a bimodal distribution. For example, a powder formed from two grades of silicon carbide with different median diameters can be used.

  Preferably, the firing temperature by high temperature treatment and all the organic components used are determined so that all said organic components are burned during firing.

  Preferably, a stabilizing agent is added to said mixture M whose instantaneous viscosity, in Pa.s, increases by at least a factor of ten when a shear rate of said stabilizing agent decreases from 100 s to 0 s -1.

  The instantaneous viscosity of said stabilizing agent, in Pa.s, increases by at least a factor of 100 when a shear rate of said stabilizing agent decreases from 100 s to 0 s -1. Advantageously, the effect on the stabilization of the foam is reinforced, which makes it possible to manufacture porous parts with thicknesses greater than 90 mm.

  The evolution of the viscosity of said stabilizing agent as a function of the shear rate of said stabilizing agent is substantially reversible. Thus, as soon as the shear rate of the mixture M increases, the influence of the stabilizing agent on the viscosity of the mixture decreases, and even may become negligible. Advantageously, the presence of the stabilizing agent therefore does not lead to a significant increase in the energy requirement.

  - In step a), said mixture M is prepared from a ceramic slip A, a premix B containing at least one gelling agent and a stabilizing agent and a premix C containing at least a foaming agent.

  Said stabilizing agent is chosen so that, at the end of step c), the viscosity of said foam is greater than 1000 Pa.s at 20 ° C. Preferably, said stabilizing agent is a hydrocolloid, preferably of plant origin, more preferably selected from the group consisting of xanthan and carrageenan or a mixture thereof. The preferred stabilizing agent is xanthan. Advantageously, the hydrocolloids of plant origin are removed during the firing step by high temperature treatment of the preform, which makes it possible to obtain a fired piece of high purity.

  - The content of said stabilizing agent in said mixture M is between 0.05 and 5% by weight, preferably between 0.1 and 1%, by weight.

  Said mixture M has a water content of less than 40%, preferably less than 30%, in percentage by weight.

  - Said gelling agent is a hydrocolloid of animal or vegetable origin capable of thermoretersibly gelling said composition after foaming. Preferably, said gelling agent is gelatin. Advantageously, the gelling agent is removed during the baking step by high temperature treatment of the preform, which allows to obtain a cooked foam of high purity. More preferably, the content of gelling agent in the mixture M is between 1 and 10%, preferably between 1 and 6%, by weight.

  In step a), a plasticizer is preferably added to said mixture M in an amount by weight of between 0.25 and 1 times that of said gelling agent. More preferably, the plasticizer is chosen to burn at a temperature above the evaporation temperature of the liquid used, in step a), to suspend the ceramic powder, usually water.

  The invention finally relates to the use of a porous ceramic according to the invention or a porous ceramic manufactured by means of a process according to the invention, for catalysis supports, for the filtration of liquids or hot gases , as a diffuser (heated part passing the air / gas mixture required for combustion) in a gas burner, in a solar volumetric receiver, or as a piece of gas (cooking media).

  Other features and advantages of the present invention will become apparent on reading the following description and on examining the appended drawing in which: FIGS. 1 to 4 show, at different magnifications, images obtained by the electron microscope at Scanning on samples taken 10 to 20 mm from the surface of sintered foams made by a process such as that described in FR 0408330, using an alumina powder; Figures 5 and 6 are graphs demonstrating rheological behavior of different pre-mixtures B; FIGS. 7 to 9 show, at different magnifications, images obtained by scanning electron microscope on samples taken at 10 to 20 mm from the surface of cooked foams manufactured according to a process according to the invention; In the various figures, identical references are used to designate identical or similar objects.

  Recrystallization is a well-known phenomenon corresponding to a consolidation by evaporation of the smallest grains of silicon carbide and condensation to form the links between the larger grains.

  By recrystallized silicon carbide means recrystallized silicon carbide during the firing step by high temperature treatment of the ceramic foam.

  The size of a pore is defined as its maximum dimension. In the products of the invention, the pores have a quasi-spherical shape and the size is in this case the diameter. The size of the pores is measured by image analysis of the surface of the cooked foams.

  We will now describe a process that can be used to manufacture the ceramic foams shown in FIGS. 1 to 4 and 7 to 9.

  This process comprises the following successive steps: a) preparation of a mixture M containing a ceramic powder in suspension, at least one gelling agent and at least one foaming agent, at a mixing temperature higher than the gelling temperature of said gelling agent, b) shearing said mixture M at a foaming temperature higher than said gelling temperature until a foam is obtained; c) gelling said foam by cooling said mixture M to a temperature below the gelling temperature of said gelling agent d) drying said gelled foam so as to obtain a preform, e) curing by high temperature treatment of said preform so as to obtain a porous ceramic foam.

  In the first step a) of this manufacturing process, a mixture is prepared containing a ceramic powder in suspension, preferably in water, at least one gelling agent and at least one foaming agent. In addition, a stabilizing agent is preferably added to said mixture M.

  The procedure is preferably as follows.

  A slurry A is first prepared by dispersing the ceramic powder and a dispersant in water, according to a conventional technique, a premix B by dissolving the gelling and stabilizing agents in water at a temperature of higher than the gelling temperature of the gelling agent, a precix C by solubilization of a foaming agent in water.

  The amount of ceramic powder in the slurry A is preferably between 50 and 90% by weight, preferably between 70 and 85% by weight, of the slurry A. Preferably, the mixture M contains from 50 to 80% by weight. preferably from 60 to 70%, by weight of ceramic particles.

  The nature of the ceramic powder is adapted according to the ceramic foam to be manufactured.

  In the case of the manufacture of recrystallized silicon carbide foam according to the invention, the powder is a silicon carbide powder whose granulometric distribution is preferably bimodal. The first mode is preferably less than 5 μm, more preferably less than 3 μm, more preferably less than 1 μm, and the second mode is preferably greater than 10 μm, preferably greater than 20 μm. Such a particle size distribution advantageously makes it possible to obtain a ceramic foam having a particularly developed intergranular porosity.

  The dispersant, for example a polyacrylate surfactant, is selected to be effective at the mixing temperature.

  The gelling agent is preferably a hydrocolloid of animal or vegetable origin capable of thermoretersibly gelling said composition after foaming, for example gelatin, carrageenan, or a mixture thereof.

  The stabilizing agent can be any. Preferably, it has the property of having a viscosity which increases by at least a factor of ten when the gradient of its shear rate decreases from 100 s to 0 s -1. Preferably, the stabilizing agent is chosen so that the viscosity of the mixture M during shearing is slightly increased because of its incorporation into this mixture.

  Advantageously, it is thus possible to use the mixers used according to the prior art without the need to increase the water content of the mixture. The increase in the water content would indeed cause problems during the subsequent drying.

  Preferably, the stabilizing agent is chosen to be reversible. More preferably, the stabilizing agent and the gelling agent are chosen so that the viscosity of the gelled foam is greater than or equal to that of a gelled foam obtained from a mixture identical to the mixture M but which does not would contain no stabilizing agent, and the viscosity of the foam obtained between steps b) and c) (before gelling and after the end of the foaming operation) is greater than or equal to that of a gelled foam obtained from of a mixture identical to the mixture M but which does not contain a gelling agent.

  Preferably the stabilizing agent and the gelling agent are selected so that they do not substantially interact with one another. The combinations of gelatin on the one hand and xanthan and / or carrageenan, preferably xanthan alone, are most preferred.

  Xanthan and carrageenan are physical gels, that is to say having a reversible network structure in which the bonds are formed by physical interaction (crystallization, helix formation, vitrification ...). For example, the xanthan molecule conforms as a single, double or triple helix that interacts with other molecules to form a complex network. In solution in water weakly bonded aggregates are formed. These aggregates can be dissociated by the shearing action.

  Preferably, at least one plasticizing agent, preferably in liquid form at the preparation temperature of the mixture M, preferably a polyol, more preferably glycerol, is added to the premix B. The plasticizer is chosen to present good compatibility with the gelling agent, that is to say an ability to remain inside the mixture M and thus resist migration and solvents, without causing phase separation. Preferably, the content of plasticizer is between 0.25 and 1 times that of the gelling agent.

  The foaming agent, preferably a soap, is preferably added in the pre-mix C in a proportion of between 55 and 75% by weight of said premix. More preferably, the amount of foaming agent is determined so that its content in the mixture M is between 1 and 3% by weight.

  The mixture M can also contain one or more thermoplastic binders usually used in ceramics.

  The slurry A and the pre-mix C are then added to the pre-mix B with mechanical stirring, the temperature, said mixing temperature being maintained higher than the gelling temperature of the gelling agent. Preferably, the pre-mixtures A, B and C are mixed immediately after their preparation to form the mixture M. The stabilizing agent leads to aqueous solutions having a stable viscosity in the temperature range used.

  The pH of the premix B may be acidic, basic or neutral and is preferably selected to allow good dispersion with premix A. In step b), the mixture M is sheared so as to foam. Shear may result from mechanical agitation, gas insufflation, or any combination of these two techniques. In the case of gas insufflation, it is preferred to use a membrane provided with calibrated holes.

  During this step b), if the viscosity of the stabilizing agent is reversible under the effect of shear, the shear reduces the viscosity of the mixture.

  Step b) is carried out at a temperature above the gelling temperature of the gelling agent, for example at the mixing temperature.

  After the ceramic foam has been obtained, the shear is cut and then the foam is poured into a mold.

  The presence of the stabilizer immediately increases the viscosity of the mixture due to the end of the shear. The stabilizing agent therefore stiffens the structure of the foam even though the temperature remains above the gelation temperature. Any sagging of the foam on itself before the gelling step is thus avoided and it becomes possible to manufacture stable foams having a thickness of more than 60 mm, and even more than 80 mm.

  In step c) the foam is cooled or allowed to cool to a temperature below the gelling temperature of the gelling agent, preferably to room temperature (10-25 ° C). Gelation advantageously makes it possible to obtain a gelled foam that is sufficiently rigid to be handled without degrading. The process is therefore well suited to industrial implementation.

  The gelled foam is then dried at ambient temperature, and then preferably baked at a temperature of 100 to 120 ° C. so as to obtain a preform that can be fired. The preform, or raw, can advantageously have a thickness of up to 80 mm, and even 150 mm.

  To manufacture ceramic foams of the type shown in FIGS. 1 to 4, the preform is calcined under air, at its sintering temperature, namely 1200-1500 C for cordierite and 1400-2300 C for alumina, mullite or zirconia or silicon carbide. Possible conditions for the high-temperature sintering of silicon carbide are described, for example, in FR 2 313 331.

  The conditions for baking in step e) making it possible to manufacture ceramic foams according to the invention, of the type of those represented in FIGS. 7 to 9, are well known, the recrystallization of silicon carbide being an old technique. For example, reference may be made to US Pat. No. 2,964,823 issued Dec. 20, 1960, or to Finely Layered Recrystallized Silicon Carbide for Diesel Particulate Filters, Process Engineering, cfi / Ber. DKG 81 (2004) n 10.

  Preferably, the preform is calcined under a non-oxidizing atmosphere, preferably under a neutral atmosphere, more preferably under argon, at a temperature which makes it possible to obtain recrystallized silicon carbide, preferably between 1800 and 2500 ° C., preferably between 2050 and 2050 ° C. and 2350 C.

  Other conditions of cooking by high temperature treatment are possible, provided that they lead to recrystallization of silicon carbide.

  After stoving and before or after curing by high temperature treatment, the foam can be machined to obtain a piece of the desired dimensions.

  The porous ceramic obtained by firing by high temperature treatment of the preform, of the type shown in FIGS. 7 to 9 or in FIGS. 1 to 4, is an open-structure and interconnected foam. It advantageously has a density after firing of between 8 and 50% of the theoretical density of the material (equal to 4 grams per cubic centimeter for alumina and 3.2 grams per cubic centimeter for recrystallized silicon carbide), ie 50 at 92% porosity.

  The size of the cell pores generally varies between 10 and 2000 μm.

  The foam has a plurality of cells connected to other adjacent cells by windows 12. A cell on the surface of the porous ceramic foam still has one or more openings 16 facing out. Interconnection porosity is the porosity created by the interconnection windows 12 between the cells and the openings 16 of the surface cells.

  Reference is now made to Figures 5 and 6.

  Figure 5 shows the evolution of the viscosity of three premixes B1, B2 and B3, provided as examples.

  B1 contains 87% water, 7.4% glycerine, 4.8% polyvinyl alcohol and 0.8% xanthan.

  B2 contains 77.3% water, 6.5% glycerin, 4.2% polyvinyl alcohol and 12% gelatin.

  B3 contains 76.7% water, 6.5% glycerine, 4.2% polyvinyl alcohol, 11.9% gelatin and 0.7% xanthan.

  These premixes are prepared at 80 ° C. and then kept at 40 ° C. for 3 hours.

  The viscosity of these mixtures, maintained at rest, is then measured as a function of their temperature by lowering the temperature by 1 ° C. per minute, from 45 ° C. (close to the temperature of preparation of the mixture M) to 10 ° C. measurement is carried out with a Bohlin CVO 120 viscometer in plane cone (diameter 40mm, angle 4) oscillating at 1 Hz with a deformation of 0.01. The results of test A are shown in FIG.

  FIG. 5 shows that xanthan alone (curve B1) does not make it possible to vary the viscosity of the mixture as a function of temperature. Xanthan is therefore not a thermoreversible gel, unlike gelatin (curve B2). Xanthan therefore does not appear to be suitable for varying the viscosity of a ceramic mixture as a function of temperature. It therefore appears particularly inappropriate to gel a ceramic foam during its cooling.

  It also appears in Figure 5 that xanthan alone does not achieve a high viscosity capable of freezing a ceramic foam. On the contrary, gelatin makes it possible to reach satisfactory viscosities at room temperature (10-25 C), between 200 and 500 times higher than those reached with xanthan alone.

  Finally, it appears in FIG. 5 that the presence of xanthan does not hinder the gelling action of gelatin, the effect of xanthan being negligible at temperatures below 25 C. The combination of xanthan and gelatin has not apparently no technical effect at temperatures below 25 C and therefore seems to be able to lead to an increase in manufacturing costs.

  In FIG. 6, the instantaneous viscosities of B1, B2 and B3 at 45 C have been represented as a function of this velocity gradient (or shear rate). In order to obtain the results shown in FIG. 6, the gradient of velocity is progressively increased, then 1 minute is waited, then the gradient of velocity is decreased in a decreasing manner. During the entire duration of the test, conducted at a temperature of 45 ° C. (close to the temperature of preparation of the mixture M), the evolution of the viscosity is measured. The measurements are made with a Bohlin CVO 120 viscometer in plane cone (diameter 40mm, angle 4) in rotation.

  It is observed that in the absence of xanthan, the viscosity of the gelatin does not change or very little according to the speed gradient. On the other hand, the instantaneous viscosity of the gelatin and xanthan mixture, like that of xanthan alone, decreases very strongly when the shear increases and returns to very high values when the shear decreases, even at low levels of xanthan.

  The inventors have also found that an increase or decrease in the viscosity of a product (gelatin, xanthan or mixture of xanthan and gelatin), following a change in temperature or shear rate leads, in the same way. temperature and shear conditions, to an improvement or deterioration, respectively, of the structural strength of the foam obtained by shearing a refractory composition incorporating this product.

  At the end of the shearing step b), the mixture of gelatin and xanthan incorporated into the mixture M thus leads, according to the teaching of FIG. 6, to an immediate improvement of the structural strength of the foam in proportions much higher than that which could be conferred by gelatin alone. The foam is then rigid enough not to collapse on itself. It is easier to handle. Finally, the action of the mixture of xanthan and gelatin avoids a modification of the structure (coalescence of air bubbles, segregation of the ceramic powder ...) which could lead to product heterogeneity and embrittlement.

  The temperature of the foam then decreases gradually, which allows, according to the teaching of Figure 5, to further increase the structural strength of the foam in proportions much greater than that which could confer xanthan alone. The foam is then rigid enough to be handled under industrial conditions without deterioration.

  The actions of gelatin and xanthan thus complement each other perfectly for the implementation of the process according to the invention, in particular for the manufacture of large parts.

  Reference is now made to FIGS. 7 to 9 which show scanning electron microscope photos of a recrystallized ceramic carbide foam with intergranular porosity, corresponding to example 21 according to the invention described below.

  This foam was obtained in the particular case where the ceramic powder used in step a) of the process described above is a powder comprising at least 95% by weight of silicon carbide, and where the baking by high temperature treatment is performed under conditions allowing the recrystallization of silicon carbide.

  These conditions are well known to those skilled in the art.

  It may be noted that the ceramic walls delimiting the cells are not of the same nature in FIGS. 1-4 and 7-8. Indeed, the walls of alumina (Figures 1 to 4) are dense while the walls of recrystallized silicon carbide (Figures 7 to 9) are porous.

  The walls 17 of recrystallized silicon carbide delimiting the cells 10 are in fact formed by agglomerating grains 18, this agglomeration leaving behind interstices 20 or intergranular pores between the grains 18.

  The grains 18 are formed by the larger particles of the silicon carbide powder used in step a). The particle size distribution of the grains 18 is therefore substantially the same as that of these larger particles.

  The walls 17 thus have a so-called intergranular porosity. The intergranular porosity is thus constituted by the interstitial spaces which necessarily creates between grains the agglomeration of these grains.

  In the particular case of recrystallized silicon carbide foams according to the invention, the intergranular porosity thus coexists with the interconnection porosity between the cells of the foam.

  As described above, the interconnection porosity is created by the cell pores, i.e. the interconnection windows 12 between the cells 10 and the openings 16 to the outside of the surface cells. The interconnection porosity is thus formed by openings whose contours are not the necessary consequence of agglomeration of grains, but result from a particular arrangement of these grains. Unlike intergranular porosity, it allows a direct (i.e. straight) connection between adjacent cells.

  Total porosity is the sum of interconnection porosity and intergranular porosity.

  In the example shown in FIGS. 7 to 9, the average cell pore size is around 100 μm while the average size of the intergranular pores is around 5 μm. The total porosity of Example 21 is 86% (density 0.43 g / cm 3) and the intergranular porosity is about 10%.

  The intergranular porosity may be modified depending on the grain size of the ceramic powder, in particular silicon carbide, used.

  The interconnection porosity may be modified depending on the foaming agent used, in particular as a function of its quantity in the initial mixture.

  The presence of an adjustable double porosity, in particular for ceramic foams based on recrystallized silicon carbide according to the invention, is particularly advantageous.

  The presence of intergranular porosity confers both a very large available surface and a low density.

  The intergranular porosity foams according to the invention are therefore effective for filtration and / or as a catalyst support, while being light.

  Their low density also implies a low thermal inertia. These foams are therefore very resistant to high temperatures and are therefore well suited to serve as cooking media. The recrystallized silicon carbide foams are particularly useful in this application because they can withstand very high temperatures. Because they efficiently conduct heat, withstand high temperatures and allow air to pass through, recrystallized silicon carbide foams are also ideally suited for forming flame diffusers in infrared burners.

  Preferably, the size of the intergranular pores is on average 10 to 100 times smaller than that of the cell pores. It is also preferable that the average cell pore size is between 80 and 120 μm, preferably around 100 μm, and / or that the average size of the intergranular pores is between 1 and 10 μm, preferably around 5 μm.

  The following nonlimiting examples are given for the purpose of illustrating the invention.

  In the examples which follow, the raw materials employed were chosen from: reactive Al 2 O 3 alumina, or a mixture of reactive aluminas, containing more than 99% Al 2 O 3, the median diameter varying from 0.1 μm to 200 pm; zirconia containing greater than 99% ZrO 2, the median diameter varying from 0.1 μm to 200 μm; dispersant prox BO3 based on ammonium polyacrylate, marketed by Synthron; GPA A0 gelatin, marketed by Wesardt International; satiaxaneTM CX90T, xanthan gum produced and marketed by SKW Biosystems; RHODOVIOL 4/125, low-hydrolysis polyvinyl alcohol (88 mol%) marketed by Rhodia PPMC; glycerine, marketed by Moulet Peinture (Avignon - France); Schäumungsmittel W53FL, dispersant based on ammonium polyacrylate, marketed by Zschimmer & Schwarz GmbH .; cordierite whose median diameter is around 4pm and containing 54% SiO2, 29% Al2O3, 13% MgO and less than 2.5% Fe2O3 + TiO2; mullite whose median diameter is around 9pm and containing 26% SiO2, 73% Al2O3, 13% MgO and less than 2.5% Fe2O3 + TiO2; silicon carbide SiC-1 whose median diameter is around 0.6 μm and containing more than 99.5% SiC; silicon carbide SiC-2 whose median diameter is around 2.5pm and containing more than 99.5% SiC; silicon carbide SiC-3 whose median diameter is around 23pm and containing more than 99.5% of silicon carbide.

  In all the following examples, the dispersant is always Prox B03, the foaming agent always W53FL, the plasticizer is glycerin and the hardener is RHODOVIOL 4/125.

  Slip A is made by dispersing the ceramic powder and the dispersant in water. The ceramic powder is alumina (Al 2 O 3) for Examples 1 to 16 and 19, mullite (Al 2 O 3 -SiO 2) for Example 17, a mixture of 80% cordierite and 20% mullite for Example 18, a mixture of 80% of alumina and 20% of zirconia for example 20 and at least two powders of silicon carbide for Examples 21 to 23 according to the following mass proportions

Example 21 22 23

  SiC-1 (%) 33 SiC-2 (%) 66 67 SiC-3 (%) 67 34 33 For Examples 21 to 23, 0.45% 1N sodium hydroxide played the role of the dispersant; the slip thus had a pH around 9.

  The premix B is made in a 55 C water bath by dissolving gelatin GPA A0 and optionally xanthan, glycerin and hardening in water.

  The premix C is made by solubilization of two thirds (by weight) foaming agent solubilized in one third of water.

  A and C are added to B in a water bath at 55 ° C. with constant mechanical stirring for 20 minutes. The proportions of the constituents of A, B and C in the final mixture M, excluding ceramic powder, are given in Table 1. The foam obtained is cast in a mold at room temperature, below 25 C. The mold has dimensions of 300mmx300mmx45mm except for example 19 for which the mold has a height of 80mm (identical section: 300mmx300mm). It is dried 24 hours at room temperature and then 24 hours at 35 C under forced ventilation to obtain the preform.

  This preform is then sintered in air for 4 hours at 1600 ° C except for Examples 1 and 2 for which the preform obtained was too friable to undergo the sintering cycle and for Examples 21 to 23 for which the preform was baked under argon for 3 hours at 2270 ° C to obtain recrystallized silicon carbide.

  It may be noted that, during high temperature treatment cooking, the dimensional shrinkage, of the order of 15 to 20%, is substantially the same in all directions.

  Some characteristics of the ceramic foams thus obtained have been reported in Tables 1 and 2.

Table 1

  . (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (i) where: the mixture M (1) g / cm 3 mPa 1 0.6 5.3 / 2.2 27.9 / / N 2 0.6 5.2 / 2.1 29.9 / / N 3 0.6 3, 1 0.1 2.2 28.6 / / 0.57 0 0 2.3 4 0.7 1.8 0.2 2.2 28.9 / / 0.48 0 0 0.7 1.0 0 , 2 2.2 29.2 / / 0.47 0 0 6 0.7 1.8 0.1 2.2 28.7 1.0 / 0.50 0 N 7 0.6 3.1 0.1 2.2 28.3 1.0 / 0.50 0 N 8 0.6 2.4 0.1 2.2 28.5 1.0 / 0.52 0 N 9 0.7 1.8 0.1 2.2 28.7 1.0 / 0.50 0 N 0.6 2.4 0.1 2.2 28.4 1.0 0.3 0.53 0 N 11 0.6 2.4 0, 1 2.2 28.4 1.0 0.1 0.54 0 N 12 0.6 1.8 0.1 2.2 29.0 1.0 0.6 0.50 0 N 13 0.6 1 , 8 0.1 2.2 28.5 1.0 0.6 0.47 0 N 14 0.6 1.8 0.1 2.2 28.3 1.0 0.6 0.94 0 N 0 , 6 1.8 0.1 2.2 28.5 1.0 0.6 0.69 0 N 4.2 16 0.6 1.8 0.1 2.2 28.3 1.0 0.6 1.08 0 N 9.3 17 0.6 1.8 0.1 1.6 28.5 1.0 0.6 0.50 0 N 18 0.6 1.7 0.1 1.5 33, 1 0.9 0.6 0.44 0 N 2.2 19 0.6 1.8 0.1 1.6 28.3 1.0 0.6 1.00 0 N 6.5 0.6 1, 8 0.05 2.2 28.5 1.0 0.6 0.55 0 N 21 0 1.7 0.1 2.1 27.2 1.0 0.6 0.43 0 N 0.4 22 0 1.8 0.1 2.3 27.2 1.0 0.7 0.40 0 N 0.5 23 0 1,9 0,1 2,2 27,3 1,0 0,7 0,81 0 N 1,7 (1) The complement is constituted by the ceramic powder After the casting, one can sometimes see an immediate freezing from stirring and / or handling (casting in a mold) ceases. By setting, it is meant that the height of the casting does not decrease and / or does not vary depending on the areas of the casting. In the table, 0 means that the freezing was observed and N means that the structure of the cast foam evolved after casting (absence of freezing).

  Finally, for some mosses we observe the presence of some cracks on the dried parts. The presence of cracks is indicated by O in Table 1, where N means that there is no crack observed. These cracks are mainly located on the edges of the pieces and have a length of a few mm to a few cm. These cracks are not critical and could be removed by machining. They are, however, troublesome.

  The density is measured according to a conventional geometric method and expressed in grams per cubic centimeter.

  The results in Table 1 highlight the advantage of adding a stabilizing agent capable of freezing the structure obtained by mechanical foaming. This produces homogeneous parts and the desired dimensions.

  The plasticizer makes it possible to obtain more flexible parts and for which the drying does not induce the formation of cracks. Furthermore, preferably, as is the case here, the plasticizer used burns at a temperature much higher than the evaporation temperature of the water. This compound will play its role even on the dried part and, during cooking, will allow to keep some moisture in the room and avoid too fast cooking and a "crust" effect on the final product. This favorable effect is particularly useful for the manufacture of large parts.

  As described above, the ceramic foams made according to the process described above, in particular the foams according to the invention, have a structure formed of nested cells delimited by ceramic walls and interconnected by windows. The size of the cellular pores is distributed in a relatively homogeneous manner between 10 and 2000 μm; less than 10% of the cell pores have a diameter less than 10 μm, 20 to 85% of the cell pores have a diameter less than 300 μm and 4 to 40% of the cell pores have a diameter greater than 450 μm.

  These foams according to the invention are also distinguished from foams known to date by a very wide range of cell pore sizes, the absence of a dominant cell pore size and a lower number of cell pores per unit area. There can be in fact less than 1500 cell pores per dm2.

  Moreover, it is observed that, as the density decreases, the increase in porosity is done by increasing the size of the cellular pores while their number decreases. More than 15% of the cell pores, and sometimes even at least 30% of the cell pores, have a diameter greater than 300 μm.

  On the contrary, the ceramic foams known today have a pore size that varies little with density and generally less than 300 μm for 90% of the pores; the decrease in density is then accompanied by an increase in the number of pores.

  The presence of large cellular pores of the foams according to the invention of Table 1 also makes it possible to obtain products having a very low pressure drop, which is particularly advantageous in certain applications, for example for gas filtration.

  These foams are also remarkable in that they have a very high chemical purity. This purity is related to the purity of the ceramic raw materials used since all the organic components used are burned during the baking process at high temperature.

  As now clearly apparent, the invention provides a porous baked ceramic foam which has, because of the specific structure of the walls which delimit the cells, a surface exposed to the considerable external environment. This new microstructure of the cell walls thus opens up very wide development perspectives.

  Of course, the present invention is not limited to the embodiments described and shown as illustrative and non-limiting examples.

  In particular, the intergranular porosity ceramic foam according to the invention is characterized by a particular microstructure, in particular a total porosity of between 50 and 92% and an intergranular porosity of at least 5%. To the inventors' knowledge, a fired ceramic foam having this microstructure, in particular a recrystallized silicon carbide fired foam, is not known from the state of the art. The invention therefore relates to any ceramic foam cooked independently of the process used to manufacture it.

  In the preferred embodiment of the invention, this microstructure results from a recrystallization of silicon carbide during the firing step by high temperature treatment. Other ceramic materials are however also possible.

Claims (16)

  1. Porous fired ceramic foam having a total porosity of 50 to 92% and an intergranular porosity of at least 5%.   The porous ceramic foam of claim 1 comprising recrystallized silicon carbide.   Porous ceramic foam according to claim 2, comprising at least 95% by weight of recrystallized silicon carbide.   Porous ceramic foam according to any one of the preceding claims having an intergranular porosity of at least 8%.   Porous ceramic foam according to claim 2 having an intergranular porosity of at least 10%.   A porous ceramic foam according to any one of the preceding claims, wherein the intergranular porosity is formed by pores less than 10 μm in size.   A porous ceramic foam according to any one of the preceding claims, wherein the size of the intergranular pores is on average 10 to 100 times smaller than that of the cell pores.   A porous ceramic foam according to any one of the preceding claims, wherein the average cell pore size is between 80 and 120 μm and / or the average size of the intergranular pores is between 1 and 10 μm.   A porous ceramic foam according to any one of the preceding claims, wherein more than 15% of the cell pores have a size greater than 300 μm and / or less than 10% of the cell pores have a size less than 10 μm and / or 20% 85% of the cell pores have a size less than 300 μm and / or 4 to 40% of the cell pores have a size greater than 450 μm, the percentages being percentages by number.   10.Process for manufacturing a porous ceramic foam according to any one of the preceding claims, comprising the following successive steps: a) preparation of a mixture M containing a ceramic powder in suspension, at least one gelling agent and at least one foaming agent, at a mixing temperature higher than the gelling temperature of said gelling agent, b) shearing said mixture M at a foaming temperature higher than said gelling temperature, until a foam is obtained, c) gelling said foam by cooling said mixture M to a temperature below the gelation temperature of said gelling agent, d) drying said gelled foam to obtain a preform, e) baking said preform so as to obtain a porous ceramic foam characterized by that the mixture M comprises silicon carbide and that the cooking in step e) is effe ctuée under conditions allowing consolidation by evaporation-recrystallization mechanism of said silicon carbide.   11.The method of claim 10, wherein the ceramic powder used in step a) has more than 95% by weight of silicon carbide.   12. A method according to any one of claims 10 to 11, wherein the particle sizes of the ceramic powder implemented in step a) are distributed in a bimodal distribution.   13.Procédé according to any one of claims 10 to 12, wherein, adding to said mixture M a stabilizing agent whose instantaneous viscosity, in Pa.s, increases by at least a factor of ten when a shear rate of said agent stabilizer decreases from 100 s to 0 s -1.   The method of claim 13 wherein said stabilizing agent is selected from the group consisting of xanthan and carrageenan or a mixture thereof.   15. A process according to any one of claims 10 to 14 wherein, in step a), a plasticizer is added to said mixture M in an amount by weight of between 0.25 and 1 times that of said gelling agent.   16.Use of a porous ceramic foam according to any one of claims 1 to 9 or a porous ceramic foam manufactured by a process according to any one of claims 10 to 15 for thermal insulation, in particular in the ovens, for catalytic supports, for the filtration of liquids or hot gases, as a diffuser in a gas burner, in a solar volumetric receiver, or as a gasifier room.