Ceramic foam

Abstract

A sintered ceramic foam that has a total porosity of greater than 60% by volume and the following phase composition, in mass percent based on the crystallized phases: 25 to 55% mullite, 20 to 65% corundum, 10 to 40% zirconia, mullite, corundum and zirconia together representing more than 80% of the mass of the crystallized phases. Also, a furnace that has a thermal insulator such ceramic foam.

Claims

1. A furnace having a thermal insulator comprising a sintered ceramic foam having cell pores, a total porosity of greater than 60% and less than 90% by volume and the following phase composition, in mass percent based on the crystallized phases: >45% and ?55% mullite, >25% of corundum, >20% and <30% zirconia, mullite, corundum and zirconia together representing more than 80% of the mass of the crystallized phases, the glassy phase representing less than 5% by mass of the ceramic foam, said sintered ceramic foam containing ZrO.sub.2 and Al.sub.2O.sub.3 in amounts such that a ZrO.sub.2/Al.sub.2O.sub.3 mass ratio is between 0.20 and 0.50, the ceramic foam resulting from sintering a mixture of mineral powders chosen from powders of alumina, mullite, mullite-zirconia, zirconia, zircon, and silica.

2. The furnace as claimed in claim 1, the ZrO.sub.2/Al.sub.2O.sub.3 mass ratio being between 0.25 and 0.40.

3. The furnace as claimed in claim 1, in which said sintered ceramic foam contains SiO.sub.2 in an amount such that a SiO2/Al.sub.2O.sub.3 mass ratio is at most 0.30.

4. The furnace as claimed in claim 3, in which the SiO.sub.2/Al.sub.2O.sub.3 mass ratio is at most 0.25.

5. A process for manufacturing a furnace according to claim 3, said process comprising the following steps: A. Preparing a first mixture comprising a mixture of: refractory oxide particles selected so as to obtain, at the end of step F., said ceramic foam, the refractory oxide particles representing more than 50% by mass of the first mixture; a liquid containing a wetting agent and/or a dispersant; B. Independently of step A., preparing a second mixture, comprising a gelling agent, a foaming agent, and preferably a plasticizer C. Incorporating the first and second mixtures in a thermostatically controlled reactor and foaming by mechanical stirring so as to obtain a foamed mixture; D. Pouring the foam mixture into a mold at a temperature below 40? C.; E. Drying the preform in order to obtain a preform having a residual water content of less than 1.5%; F. Sintering in an oxidizing atmosphere.

6. The process as claimed in claim 5, wherein: at step A, the first mixture consists of a mixture of said refractory oxide particles and said liquid; at step B, the second mixture consists of gelatin or a gelatin derivative, a foaming agent, and preferably glycerin or a glycerin derivative at step C, the mechanical stirring lasts longer than 10 minutes, at step D, the foam mixture is poured at a temperature at step F, sintering is performed in air, at a temperature above 1600? C. and below 1750? C., fora period of more than 2 hours.

7. The furnace as claimed in claim 1, the ceramic foam having a total porosity of between 65% and 85%.

8. The furnace as claimed in claim 1, wherein mullite, corundum and zirconia together represent more than 95% of the mass of the crystallized phases.

9. The furnace as claimed in claim 1, wherein the mullite content is less than 50%.

10. The furnace as claimed in claim 1, wherein the corundum content is greater than 30%.

11. The furnace as claimed in claim 1, wherein more than 60% of the zirconia, by mass percent, is in monoclinic crystalline form.

12. The furnace as claimed in claim 1, the ceramic foam having a chemical composition such that, in mass percentages based on the oxides and for a total of 100%: Al2O3: <70%; SiO2: ?25%; ZrO2: ?40%; Fe2O3+MnO+B2O3+Na2O+K2O: less than 2%; CaO+MgO+Y2O3+Ce2O3: less than 10%; optional other refractory oxides: supplement to 100%.

13. The furnace as claimed in claim 12, wherein: CaO+MgO+Y2O3+Ce2O3: less than 3%.

14. The furnace as claimed in claim 1, the ceramic foam having a macroporosity and an open porosity which represents more than 60% of the macroporosity.

15. The furnace as claimed in claim 1, the ceramic foam having a mean grain size greater than or equal to 5 micrometers and/or less than 200 micrometers.

16. The furnace as claimed in claim 1, the ceramic foam having a mean grain size less than 100 micrometers.

17. The furnace as claimed in claim 1, in which the mean size of the cell pores is between 80 and 1000 ?m.

18. The furnace as claimed in claim 1, in which the mean size of the cell pores is between 150 and 700 ?m.

19. The furnace as claimed in claim 1, in which the mean size of the cell pores is between 200 and 500 ?m.

20. The furnace as claimed in claim 1, the mean wall thickness of walls delimiting the cells pores being between 30 and 300 micrometers.

Description

DETAILED DESCRIPTION

(1) Phase Composition

(2) The mullite content is preferably greater than 35%, preferably greater than 40%, or even greater than 45%, and/or less than 55%, preferably less than 50%.

(3) The corundum content is preferably greater than 25%, preferably greater than 30%, and/or less than 60%, preferably less than 55%, preferably less than 50%, preferably less than 40%.

(4) The zirconia content is preferably greater than 15%, preferably greater than 20%, and/or less than 35%, preferably less than 30%.

(5) Ceramic foam preferably comprises: 40 to 60%, preferably 40 to 55% mullite, 30 to 40% corundum, 15 to 30% zirconia.

(6) Preferably still, more than 40%, preferably more than 60%, preferably more than 80%, preferably more than 90%, preferably substantially 100% of the zirconia, in percent by mass, is in monoclinic crystalline form.

(7) Zirconia which is not in monoclinic crystalline form is preferably stabilized with yttrium, and/or cerium, and/or calcium and/or magnesium.

(8) The glassy or amorphous phase preferably represents less than 5% by mass of the ceramic foam; preferably the foam does not contain a glassy phase detectable by X-ray diffraction analysis.

(9) The composition of the phases was obtained by X-ray diffraction. The diffractograms of the crystallized phases can be collected with a D5000 type diffractometer and the data were analyzed qualitatively with the EVA software and the ICDD2016 database.

(10) Chemical Composition

(11) Preferably, the ceramic foam has the following chemical composition, in mass percentages based on the oxides: Al.sub.2O.sub.3: 50 to 80%; SiO.sub.2: 5 to 25%; ZrO.sub.2: 10 to 40%; Fe.sub.2O.sub.3+MnO+B.sub.2O.sub.3+Na.sub.2O+K.sub.2O: less than 2%, preferably less than 1%; Sum of the oxides of calcium, magnesium, yttrium and cerium: less than 10%, preferably less than 5%, preferably less than 3%; Other refractory oxides: supplement to 100%.

(12) A sum of oxides, for example Fe.sub.2O.sub.3+MnO+B.sub.2O.sub.3+Na.sub.2O+K.sub.2O, refers to the total content of these oxides, but does not imply that they are all present.

(13) Of course, the phase composition of a ceramic foam according to the invention implies limitations on the composition, and in particular on the total content of Al.sub.2O.sub.3+SiO.sub.2+ZrO.sub.2.

(14) The Al.sub.2O.sub.3 content is preferably greater than 55%, preferably greater than 60%, and/or less than 70%.

(15) Preferably, the SiO.sub.2 content is greater than 7%, preferably greater than 10% and/or less than 20%.

(16) Preferably, the ZrO.sub.2 content is greater than 15%, and/or less than 30%, preferably less than 25%.

(17) Preferably, the total Al.sub.2O.sub.3+SiO.sub.2+ZrO.sub.2 content is greater than 90%, preferably greater than 95%, preferably greater than 98%, in mass percentages based on the oxides.

(18) The SiO.sub.2/Al.sub.2O.sub.3 mass ratio is preferably between 9 and 30, preferably between 10 and 25.

(19) The ZrO.sub.2/Al.sub.2O.sub.3 mass ratio is preferably between 15 and 80, preferably between 20 and 50, more preferably between 25 and 40.

(20) Preferably, the content of TiO.sub.2, which belongs to the other oxides, is less than 1% or even less than 0.5%.

(21) The other oxides are oxides other than Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, Fe.sub.2O.sub.3, MnO, B.sub.2O.sub.3, Na.sub.2O, K.sub.2O, CaO, MgO, Y.sub.2O.sub.3 and Ce.sub.2O.sub.3.

(22) Preferably, the ceramic foam contains less than 1% chromium oxide Cr.sub.2O.sub.3, which is part of the other oxides. Preferably it does not contain chromium oxide.

(23) The other oxides are preferably less than 20%, preferably less than 15%, preferably less than 8%, or even less than 5%, less than 3.0%, less than 2.0%, less than 1.0%, less than 0.5%. In one embodiment, the other oxides are impurities.

(24) Oxides represent more than 90%, preferably more than 95%, preferably more than 99%, preferably substantially 100% of the mass of the ceramic foam.

(25) The mass content of the above-mentioned oxides is typically determined by X-ray fluorescence and/or inductively coupled plasma (ICP).

(26) The organic matter content, measured conventionally by loss on ignition at 750? C. for 30 minutes in air, is preferably less than 0.5%, or even less than 0.1%, in mass percentage based on the foam.

(27) Microstructure

(28) The ceramic foam has a plurality of cells 10 (see FIG. 1), the majority of these cells being connected to other adjacent cells by windows 12. A cell on the surface of the porous ceramic foam also usually has one or more openings to the outside.

(29) The walls 17 delimiting the cells 10, made up of particles agglomerated by sintering, have an intergranular porosity. They are in fact formed by agglomeration of particles 18, this agglomeration leaving interstices 19 or intergranular pores, between the particles 18.

(30) The intergranular porosity can be modified according to the particle size of the ceramic powder used. The mean size of the intergranular pores is preferably less than 10 ?m The intergranular porosity, preferably greater than 1%, and/or less than 10%.

(31) The volumes delimited by the walls 17 define a macroporous porosity. The size of the cell pores, or macropores, generally ranges from 10 to 2000 ?m. The mean size of the cell pores is preferably 10 to 100 times larger than that of the intergranular pores, preferably between 80 and 1000, preferably between 100 and 1000 micrometers, preferably between 150 and 700 ?m, preferably between 200 and 500 ?m, preferably about 400 ?m.

(32) Intergranular porosity thus coexists with macroporosity, the total porosity being the sum of macroporosity, or macroporous porosity, and intergranular porosity. Macroporosity is thus formed by volumes whose limits are not the necessary consequence of grain agglomeration, but result from a particular arrangement of these grains and the foaming process.

(33) Macroporosity includes closed volumes or pores, i.e. defined by cells not connected with other cells, and open volumes or pores, i.e. interconnected with other pores. A ceramic foam according to the invention has a total porosity equal to the sum of intergranular porosity, open macroporosity and very high closed macroporosity. Total porosity is typically between 65 and 95% by volume.

(34) Preferably, the total porosity is greater than 60%, greater than 70%, greater than 75%, greater than 78% and/or less than 90%, preferably less than 85%, by volume.

(35) The ceramic foam has a total macroporosity (sum of open and closed macroporosities) of greater than 70%, preferably greater than 75% or even greater than 80%, and/or less than 95% or even less than 90%, in volume percent.

(36) Open porosity preferably represents more than 60%, preferably more than 70%, preferably more than 80%, preferably more than 90%, preferably more than 95% of the macroporosity.

(37) The mean wall thickness is typically between 30 and 300 micrometers, preferably between 45 and 200 micrometers, preferably between 45 and 100 micrometers.

(38) The interconnection windows 12 preferably have, on average, an equivalent diameter greater than 1/100 and/or less than ? of the mean equivalent diameter of the cells 10.

(39) The mean grain size is preferably greater than or equal to 5 micrometers and/or less than 200 micrometers, preferably less than 100 micrometers.

(40) More than 90%, preferably more than 95%, preferably substantially 100% by number, of the grains are preferably alumina grains, in particular tabular alumina, and/or mullite grains (shown in gray in FIG. 2), and/or zirconia grains (shown in white in FIG. 2), and/or mullite-zirconia grains, and/or alumina-zirconia grains, and/or alumina-zirconia-silica grains.

(41) The grains are sintered or fused grains, including electrofused grains, for example alumina-zirconia or alumina-zirconia-silica.

(42) Preferably the grains contain less than 5%, preferably less than 2%, preferably less than 1%, preferably substantially no free silica.

(43) According to an embodiment, zirconia can be present as zirconia grains or as sintered or electrofused mullite-zirconia grains.

(44) These microstructures can be observed using a scanning electron microscope (SEM).

(45) Process of Manufacture

(46) A ceramic foam according to the invention can be produced in particular according to steps a) to e) above, in particular according to a process described in EP 1 329 439.

(47) Preferably, the ceramic foam is produced by a process comprising the above steps A. to F.

(48) This process is conventional, except for steps A. and B. Advantageously, this difference makes it possible to obtain a great homogeneity, even in the presence of gelling agent.

(49) In step A., the first mixture is preferably prepared at a temperature between 40 and 70? C., preferably between 50 and 60? C., preferably under continuous mechanical stirring, preferably for more than 30 minutes, preferably more than 60 minutes and, preferably less than 90 minutes.

(50) The mass quantity of dispersant is preferably between 0.1 and 5% in relation to the dry matter of the first mixture.

(51) In one embodiment, the first mixture does not contain wetting agent.

(52) The wetting and dispersing agents can be conventional.

(53) The quantity of water in the first mixture, preferably deionized, is between 10 and 25%, more preferably between 12 and 20%, or even between 13 and 18% in relation to the dry matter.

(54) In step B., the second mixture is preferably prepared at a temperature between 40 and 70? C., preferably between 50 and 65? C., under continuous mechanical stirring, for 10 minutes, or even 20 minutes, and preferably less than 30 minutes and less than 1 hour.

(55) The quantity of water in the second mixture, preferably deionized, is preferably between 5 and 15% in relation to the dry matter.

(56) In step C., mechanical stirring is preferably continued until an expansion ratio or rate of expansion greater than 2, greater than 4, or even greater than 5, is obtained.

(57) In step D., the sides of the mold in contact with the foam mixture are preferably made of Bakelite or Teflon. Preferably, the preform is demolded before drying.

(58) Preferably, the mold can be dismantled. A release agent can be used to facilitate the release operation.

(59) Alternatively, it is possible to introduce air, or more generally a gas, into the second mixture before step C. Thus, the second mixture of gelatin and foaming agent is brought in already foamed before being mixed with the first mixture containing mineral particles. Such an alternative is described in the publication Foam sprayed porous insulating refractories by V. R. Salvini, A. P. Luz and V. C. Pandolfelli Refractories Worldforum 4 (2012) [4].

(60) Alternatively, it is possible to use a foam mixer, preferably of the rotor-stator type, in which the first and second mixtures are added successively or simultaneously, air, or more generally a gas, preferably being introduced into the feedstock (first mixture+second mixture) of the foam mixer.

Examples

(61) The following non-limiting examples are given for the purpose of illustrating the invention.

(62) Manufacturing

(63) In the examples, the raw materials used were selected from: corundum Al.sub.2O.sub.3 powder of greater than 99.5% purity, with particles less than 0.2 mm in size; corundum Al.sub.2O.sub.3 powder of greater than 99.5% purity, with particles less than 50 micrometers in size; a mixture of calcined and reactive aluminas, containing more than 99% Al.sub.2O.sub.3 and having a median size of between 0.1 and 7 micrometers; a powder of mullite particles, the powder having a median size of between 7 and 15 micrometers and having the following composition: Al.sub.2O.sub.3: 72% SiO.sub.2: 26% a powder of mullite-zirconia particles, the powder having a median size of between 7 and 15 micrometers and having the following composition: Al.sub.2O.sub.3: 45% SiO.sub.2: 19% ZrO.sub.2: 35% a zirconia powder CC10, containing more than 99% ZrO.sub.2, with a median size D.sub.50 of 3 to 5 micrometers, supplied by the company SEPR; zircon sand, with a purity of ZrO.sub.2+SiO.sub.2 greater than 98%, and a median size D.sub.50 of 4 to 6 micrometers; quartz sand, with a purity higher than 98% and a median size D.sub.50 of about 3 micrometers; silica fume ERQ4, median size D.sub.50 of 0.5 micrometers, supplied by SEPR; 280 bloom20 mesh quality gelatin; glycerin of a purity greater than or equal to 99.7%; a foaming agent based on fatty alcohol sulfonate; a dispersant based on ammonium polyacrylate; an yttrium oxide powder of greater than 99% purity, having a median size of about 1.6 micrometers.

(64) The following table, Table 1, summarizes the mineral raw material compositions of the feedstocks.

(65) TABLE-US-00001 TABLE 1 Comparative examples According to the invention C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 1 2 3 4 5 Corundum powder 39.1 39.1 39.1 39.1 0 39.1 39.1 5 24 14 5 0 0 0 0 0 0-0.2 mm Corundum powder 8 8 8 8 8 8 8 7 5 3 2 8 8 8 8 7 0-50 micrometers Mullite powder 0 0 0 0 0 0 0 8 18 2 21 0 0 0 0 0 Mullite-zirconia powder 0 0 0 0 39.1 0 0 27.1 0 28 19 39.1 39.1 39.1 39.1 39.1 Mixture of calcined and 52.9 46.9 39.9 39.9 17.9 42.4 37.9 17.9 32 17.9 24 52.9 39.9 39.9 39.9 39.9 reactive aluminas Zircon sand 0 0 0 11 0 10.5 15 0 0 5 0 0 11 0 11 11 Zirconia CC10 0 0 7 0 29 0 0 29 21 30.1 23 0 0 7 0 0 Quartz sand 0 0 4 1 4 0 0 4 0 0 4 0 1 4 1 1 Silica fume ERQ4 0 6 2 1 2 0 0 2 0 0 2.0 0 1 2 1 1 Yttrium oxide 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 Total 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

(66) The ceramic foams were manufactured as follows: First and second mixtures were prepared, each in a bath thermostatically controlled at a temperature below 65? C., by adding: for the first mixture, the dispersant (+1 to +1.5% in relation to the dry mass), mineral powders and deionized water (between +14% and 16.2% in relation to the dry mass); for the second mixture, 10 to 12% deionized water, poured into a thermostatically controlled tank, then 3% gelatin (gelling agent), 2% glycerin (plasticizing agent) and finally 1 to 2% foaming agent were added, in percentages in relation to the dry matter.

(67) Each of the first and second mixtures was kept under constant mechanical stirring, using a disperser equipped with a dispersing disk, for a minimum of 20 minutes until good dispersion was obtained.

(68) The first and second mixtures were then mixed, again in a thermostatically controlled tank, for more than 20 minutes to obtain a foamy mixture.

(69) The foamy mixture was poured into a Bakelite mold at room temperature (below 25? C.) and the contents of the mold were then dried for 150 hours at a temperature below 28? C. to obtain a preform with the following dimensions: 300 mm?300 mm?60 mm.

(70) The preform was then sintered under air for 6 hours at a temperature between 1600? C. and 1750? C., to obtain a sintered ceramic foam.

(71) Table 2 provides the chemical composition of the ceramic foams obtained, in mass percentages based on the oxides.

(72) Table 3 provides the phase composition of the ceramic foams obtained, in mass percentages based on the crystallized phases (which represent substantially 100% of the phases present).

(73) Characterization of Properties

(74) Each ceramic foam was characterized as follows:

(75) Measurements of dynamic modulus of elasticity (MoE) values are determined at room temperature (20? C.) and after thermal shock, according to ASTM C 1259, using an IMCE, RFDA system23 measuring device. Heat shock consists in subjecting a sample initially at room temperature to a heat treatment consisting of a temperature rise to 1200? C., at a speed of 150? C./h, holding at this bearing temperature for 3.5 h, then quenching in air, by natural convection. The reported value is an average obtained on three samples with the following dimensions 180*40*40 mm.sup.3.

(76) Measurements of modulus of rupture (MoR) values are performed at room temperature (20? C.), and after thermal shock, according to NF EN 843-1 or ISO 14610, in a 4-point bending configuration. Heat shock consists in subjecting a sample initially at room temperature to a heat treatment consisting of a temperature rise to 1200? C., at a speed of 150? C./h, holding at this bearing temperature for 3.5 h, then quenching in air, by natural convection. The reported value is an average obtained on three samples with the following dimensions 180*40*40 mm.sup.3.

(77) Measurements of modulus of rupture (MoR) values at 1400? C. (hot MoR) are carried out according to NF EN 993-7, in a 3-point bending configuration. The reported value is an average obtained on three samples with the following dimensions 150*25*25 mm.sup.3.

(78) MoR loss (%) is equal to:
[MoR (sample before heat treatment)?MoR (sample after heat treatment)]*100/[MoR (sample before heat treatment)]

(79) This value is an indication of the resistance to thermal shock.

(80) The ratio (MoR (MPa)*1000)/MoE (GPa) is calculated from the MoR and MoE values measured on non-heat-treated samples. This ratio is an indication of the resistance to the thermal gradient.

(81) The pore volume is calculated from the measurement of geometric density and of absolute density according to the following formula:
Pore volume (%)=[absolute density?geometric density]*100/absolute density

(82) The pore volume corresponds roughly to the macroporosity.

(83) The geometric density is measured according to ISO 5016:1997 or EN 1094-4 and expressed in g/cm.sup.3. It is conventionally equal to the ratio of the mass of the sample divided by the apparent volume.

(84) The absolute density value, expressed in g/cm.sup.3, is conventionally measured by dividing the mass of a sample by the volume of the sample ground so as to substantially remove porosity. In this case, the grinding is adapted to reduce the sample to a powder with a particle size of less than 40 micrometers. Absolute density can be measured by helium pycnometry using an Accupyc II 1340 instrument from Micromeritics. The standard used by the manufacturer is ASTM C604-02 (2012).

(85) The total porosity, in percentage, is conventionally equal to 100*(1?the ratio of the geometric density divided by the absolute density).

(86) Thermal conductivity is measured according to ISO 8894-2, at 1200? C. The reported value is an average obtained from five measuring points.

(87) The results are reported in Table 4 below.

(88) TABLE-US-00002 TABLE 2 Comparative examples According to the invention C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 1 2 3 4 5 % Al.sub.2O.sub.3 99.5 93.5 86.5 86.5 43.2 89.0 84.5 47.9 73.8 48.9 54.4 78.0 65.2 65.2 65.2 64.2 % SiO.sub.2 <0.3 6.0 6.1 5.8 13.5 3.7 5.1 13.2 4.8 7.6 15.8 7.7 13.2 13.5 13.2 13.2 % ZrO.sub.2 0.0 0.0 7.0 7.3 42.6 6.9 9.9 38.2 20.8 42.8 28.9 13.7 21.0 20.7 21.0 21.0 Al.sub.2O.sub.3 (%) + SiO.sub.2 (%) + 99.8 99.5 99.6 99.6 99.3 99.6 99.5 99.3 99.4 99.3 99.1 99.4 99.4 99.4 99.4 98.4 ZrO.sub.2 (%) % Y.sub.2O.sub.3 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 1.0 Rapport SiO.sub.2/Al.sub.2O.sub.3 <0.5 6.4 7.0 6.6 31.2 4.1 6.1 27.5 6.5 15.6 29 9.9 20.2 20.7 20.2 20.5 (*100) Rapport ZrO.sub.2/Al.sub.2O.sub.3 0 0 8.1 8.4 98.5 7.8 11.7 79.7 28.1 87.6 53.1 17.5 32.2 31.8 32.2 32.7 (*100)

(89) TABLE-US-00003 TABLE 3 Comparative examples According to the invention C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 1 2 3 4 5 % Mullite 1.0 22.3 22.3 20.9 47.0 13.2 18.4 45.9 17.9 31 15 25.8 45.7 47.1 45.7 45.7 % Corundum 99.0 77.7 70.7 71.7 10.0 79.7 71.5 14.9 61.2 26 55 60.3 33.0 32.0 33.0 32.0 % Zirconia 0.0 <0.1 7.0 7.4 42.7 7.1 10.1 38.2 20.9 43.0 30 13.7 21.1 20.7 21.1 21.1

(90) TABLE-US-00004 TABLE 4 Comparative examples C1 C2 C3 C4 C5 C6 C7 C8 C9 Geometric density 0.70 0.70 0.69 0.67 0.81 0.43 0.42 0.83 0.75 (g/cm.sup.3) Absolute density 3.97 3.77 3.90 3.88 4.03 3.97 3.94 4.08 4.06 (g/cm.sup.3) Total porosity (%) 82.4 81.4 82.3 82.7 79.9 89.2 89.3 79.7 81.5 4-point MoR at 3.7 2.3 3.4 3.3 2.9 0.9 0.75 2.85 2.1 20? C. (MPa) MoR * 1000/MOE 378 348 415 413 432 377 412 425 407 MoR loss (%) after 78 68 79 76 66 NM NM 67 71 thermal shock 3-point MoR at 0.9 1.2 NM NM 2.9 NM NM NM NM 1400? C. (MPa) Thermal conductivity 0.90 0.80 NM NM 0.65 0.72 0.73 0.64 NM at 1200? C. [W/(m * K)] Comparative examples According to the invention C10 C11 1 2 3 4 5 Geometric density 0.81 0.73 0.82 0.78 0.70 0.52 0.81 (g/cm.sup.3) Absolute density 4.34 3.85 3.92 3.77 3.77 3.78 3.83 (g/cm.sup.3) Total porosity (%) 81.3 81.0 79.1 79.3 81.4 86.2 78.9 4-point MoR at 2.6 2.6 3.8 2.7 3.3 1.2 3.7 20? C. (MPa) MoR * 1000/MOE 371 419 518 474 471 447 496 MoR loss (%) after 73 69 55 59 61 64 62 thermal shock 3-point MoR at NM NM 2.9 3.0 NM 1.1 4.1 1400? C. (MPa) Thermal conductivity 0.63 NM NM 0.58 0.61 0.54 NM at 1200? C. [W/(m * K)] NM: not measured

(91) The results show an improvement in thermal shock resistance, thermal gradient resistance, mechanical strength at high temperature, with low thermal conductivity at high temperature, regardless of the precursors added to form the mullite and zirconia phases.

(92) Examples 2 and 5 are the preferred examples.

(93) As is now clear, the phase composition of a ceramic foam according to the invention improves the resistance to thermal cycling, and more generally to thermal gradients, the MoR*1000/MoE ratio being able to exceed 440, 470, 500 or even 515.

(94) Furthermore, a ceramic foam also has: a very high four-point modulus of rupture (MoR) at 20? C., which can be higher than 2.5 MPa or even 3.5 MPa for a pore volume of about 80%; a very high three-point modulus of rupture (MoR) at 1400? C., which can be higher than 3.0 MPa or even 4.0 MPa for a pore volume of about 80%; low thermal conductivity at high temperature (at 1200? C.), which can be less than 0.8 W/(m.Math.K), and even less than 0.6 W/(m.Math.K); improved thermal shock resistance, as the loss of 4-point MoR following heat treatment can be less than 65%, 60%, and even less than 55%.

(95) A ceramic foam according to the invention furthermore has a high total porosity, preferably greater than 79% and even greater than 82%. Preferably, the total porosity is less than 85%. Above 85%, the 4-point modulus of rupture decreases very significantly.

(96) A ceramic foam according to the invention is therefore perfectly suitable for use as a thermal insulator in a furnace.

(97) Finally, advantageously, a ceramic foam according to the invention can be made by direct foaming. This process makes it possible to manufacture, by casting, parts of various shapes and/or with a thickness greater than 50 mm.

(98) Of course, the invention is not however limited to the described embodiments, provided for illustrative purposes only.