Ceramic foam

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 2 and Al 2 O 3 in amounts such that a ZrO 2 /Al 2 O 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 2 /Al 2 O 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 2 in an amount such that a SiO2/Al 2 O 3 mass ratio is at most 0.30.

4. The furnace as claimed in claim 3 , in which the SiO 2 /Al 2 O 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 μm.

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

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

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