Fused spinel-zirconia grains and refractory product obtained from said grains

Abstract

Fused grains, in which the grains include a matrix of the zirconia-spinel eutectic coating inclusions composed essentially of a zirconia phase or of a spinel phase, the grains exhibit the following overall chemical composition, as percentages by weight expressed in the form of oxides: more than 45.0% and less than 95.0% of ZrO.sub.2, more than 3.0% and less than 40.0% of Al.sub.2O.sub.3, more than 1.0% and less than 20.0% of MgO, wherein ZrO.sub.2, Al.sub.2O.sub.3 and MgO together represent at least 95.0% of the weight of the grains.

Claims

1. Fused grains, in which: said grains comprise a matrix of a zirconia-spinel eutectic coating inclusions composed essentially of a zirconia phase or of a spinel phase, said grains exhibit the following overall chemical composition, as percentages by weight expressed in the form of oxides: more than 45.0% and less than 95.0% of ZrO.sub.2, more than 3.0% and less than 40.0% of Al.sub.2O.sub.3, more than 1.0% and less than 20.0% of MgO, ZrO.sub.2, Al.sub.2O.sub.3 and MgO together represent at least 95.0% of the weight of said grains.

2. The fused grains as claimed in claim 1, the chemical composition of which comprises more than 68% by weight of ZrO.sub.2.

3. The fused grains as claimed in claim 1, the chemical composition of which comprises less than 25% by weight of Al.sub.2O.sub.3.

4. Fused grains, in which: said grains comprise a matrix of a zirconia-spinel eutectic coating inclusions composed essentially of a zirconia phase or of a spinel phase, said grains exhibit the following overall chemical composition, as percentages by weight expressed in the form of oxides: more than 45.0% and less than 95.0% of ZrO.sub.2, more than 3.0% and less than 40.0% of Al.sub.2O.sub.3, more than 1.0% and less than 20.0% of MgO, ZrO.sub.2, Al.sub.2O.sub.3 and MgO together represent at least 95.0% of the weight of said grains, and in which the Al.sub.2O.sub.3/MgO ratio by weight is between 1.0 and 5.0.

5. The fused grains as claimed in claim 1, in which ZrO.sub.2, Al.sub.2O.sub.3 and MgO together represent more than 98.0% of the weight of said grains.

6. The fused grains as claimed in claim 1, additionally comprising, on the basis of the oxide, more than 0.2% and less than 4% by weight of Y.sub.2O.sub.3.

7. The fused grains as claimed in claim 1, additionally comprising, on the basis of the oxide, more than 0.2% and less than 4% by weight of CaO.

8. The fused grains as claimed in claim 1, in which more than 50% of the inclusions essentially composed of zirconium oxide exhibit a greater dimension of less than 500 micrometers.

9. The fused grains as claimed in claim 1, in which the spinel phase represents between 5% and 50% by weight of said grains.

10. The fused grains as claimed in claim 1, in which the ZrO.sub.2-spinel eutectic represents between 10% and 80% by volume of said grains.

11. A refractory ceramic material or product obtained by sintering or consolidation of fused grains, or of a mixture of starting materials comprising fused grains, as claimed in claim 1.

12. A method comprising manufacturing a refractory material with fused grains, in which: said grains comprise a matrix of a zirconia-spinel eutectic coating inclusions composed essentially of a zirconia phase or of a spinel phase, said grains exhibit the following overall chemical composition, as percentages by weight expressed in the form of oxides: more than 45.0% and less than 95.0% of ZrO.sub.2, more than 3.0% and less than 40.0% of Al.sub.20.sub.3, more than 1.0% and less than 20.0% of MgO, ZrO.sub.2, Al.sub.2O.sub.3 and MgO together representing at least 95.0% of the weight of said grains.

13. The method as claimed in claim 12, wherein the refractory material is a refractory material for metallurgy.

14. The method as claimed in claim 12, wherein said refractory material is obtained by sintering starting materials comprising said fused grains or consisting of said fused grains.

15. The method as claimed in claim 12, wherein said refractory material is obtained by consolidation of starting materials comprising said fused grains or consisting of said fused grains.

16. The method as claimed in claim 12, wherein said refractory material is obtained by sintering or consolidation of starting materials comprising a powder of zirconia and a powder of said fused grains, said powder of zirconia and said powder of fused grains together representing more than 90% by weight of said starting materials.

17. The fused grains as claimed in claim 4, in which the Al.sub.2O.sub.3/MgO ratio by weight is between 1.5 and 3.0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a first electron microscopy photograph of the microstructure of a fused grain according to the invention.

(2) FIG. 2 is a first electron microscopy photograph of the microstructure of the same fused grain as in FIG. 1 but at higher magnification.

(3) The following definitions are given:

(4) Refractory materials are understood to mean, in accordance with the standard ISO 836:2001 (point 107), a nonmetallic material or product (but not excluding the materials or products containing a certain proportion of metal), the chemical and physical properties of which allow it to be employed in a high-temperature environment. For example, such high temperatures can be greater than 600 C., in particular greater than 800 C., indeed even greater than 1000 C.

(5) zirconia refers generally to zirconium oxide ZrO.sub.2; it generally comprises a small amount of hafnium oxide HfO.sub.2, in the form of an unavoidable impurity, it being possible for this amount to range up to 2% of the total amount of zirconia. In the overall chemical composition of the grains according to the invention, in particular as described above, the ZrO.sub.2 percentages correspond in particular to the summed amount of zirconium oxide and of the unavoidable impurity HfO.sub.2.

(6) In contrast, zirconia phase or zirconium oxide phase refers to a phase consisting of zirconia (including the unavoidable impurities, in particular HfO.sub.2) or of zirconia which is partially or completely stabilized, in particular by magnesium or yttrium.

(7) Spinel refers to the compounds formed by the reaction between magnesium oxide and alumina, often expressed in the MgAl.sub.2O.sub.4 form, the crystallographic structure of which can be described as a stack of cubic type of O.sup.2 ions in which half of the octahedral sites are occupied by Al cations and a quarter of the tetrahedral sites are occupied by Mg cations. Such a crystallographic structure can also accept an excess of Al or Mg cations in solid solution while remaining a spinel compound within the meaning of the present invention.

(8) In other words, a spinel phase in a fused grain according to the present invention can substantially deviate from the conventional spinel formulation MgAl.sub.2O.sub.4, that is to say from a 1:1 stoichiometry of the molar ratio of Al.sub.2O.sub.3 to MgO.

(9) Zirconia-spinel eutectic refers to a microstructure obtained from the eutectic point corresponding, in the ZrO.sub.2/MgAl.sub.2O.sub.4 pseudo-ternary diagram, to a point with the composition in the region of 59 mol % of zirconia and of 41 mol % of spinel MgAl.sub.2O.sub.4, the melting point of which is in the vicinity of 1830 C. (invariant point of the phase diagram for which the liquid to solid reaction is complete).

(10) The crystallographic structure of such a eutectic is visible in the two electron microscopy photographs given in the appended FIGS. 1 and 2.

(11) FIG. 1 is a first electron microscopy photograph of the microstructure of a fused grain according to the invention.

(12) FIG. 2 is a first electron microscopy photograph of the microstructure of the same fused grain as in FIG. 1 but at higher magnification.

(13) At very high magnification (FIG. 2), it is observed that the matrix made of the preceding eutectic composition and exhibiting a crystallographic structure in which the zirconia phase is very finely dispersed, in the form of short rods or fibers, in the spinel phase. Such a structure is characteristic of a eutectic phase obtained from two crystalline types.

(14) Fused grains refers to grains obtained by a manufacturing process comprising at least a stage of melting, a stage of solidification and a stage of dividing, in particular by grinding, by molding or any other known equivalent means.

(15) A powder according to the invention is an assembly of grains according to the invention, the particle size of which is suited to a specific use.

(16) In other words, a use according to the invention is generally carried out starting from a powder consisting of an assembly of grains as described above, the particle size of which is suited to the manufacture of said refractory material.

(17) Melting a mixture of precursors or oxides refers to a heat treatment at a temperature sufficiently high for all the constituents of the mixture to be found in the molten (liquid) state.

(18) Conventionally, in the field of ceramics, sintering an assembly of grains refers to a heat treatment which makes possible the joining and the growth of their contact interfaces by movement of the atoms inside and between the grains, within the meaning indicated in the standard ISO 836:2001 (point 120).

(19) According to the invention, the sintering temperature of the fused grains is normally between 1100 C. and 1500 C., in particular between 1300 and 1500 C.

(20) Alternatively, consolidation is understood to mean a heat treatment of the grains at a more moderate temperature suitable for the simple shaping of a ceramic component, without strong bonds, however, between the interfaces of the grains, in contrast to the sintering process described above, it being possible for the bonding to be provided by a binder, for example a phenolic resin.

(21) According to the invention, the consolidation temperature of the fused grains is normally between 500 C. and 1100 C., in particular between 600 C. and 1000 C.

(22) The size of the grains is measured according to the well known techniques of laser particle sizing up to 20 micrometers and then by conventional sieving techniques above 20 micrometers.

(23) A better understanding of the invention and its advantages will be obtained on reading the nonlimiting examples which follow. In the examples, all the percentages are given by weight.

EXAMPLES

(24) Comparative example 1 is a powder of zirconia partially stabilized with magnesium oxide. This example is characteristic of the starting materials used today for the manufacture of refractory components in the field of metallurgy, in particular for the pouring of steels.

(25) Comparative example 2 is a mixture of the partially stabilized zirconia powder used in example 1 with a spinel powder comprising approximately 72% of Al.sub.2O.sub.3 and 28% of MgO, without additional heat treatment.

(26) Examples 3 and 4 according to the invention are prepared from the necessary proportions of the following starting materials: Alumina AR75 comprising more than 98% of Al.sub.2O.sub.3, sold by Alcan, MgO sold by the company Altichem, comprising more than 98% of MgO, Zirconia with a degree of purity of greater than 98%.

(27) The mixture of the initial reactants thus obtained according to examples 3 and 4 is electrically melted with an electric arc furnace, under air. The molten mixture is poured as an ingot. The cooled ingot obtained is ground and sieved in order to obtain a powder of fused grains, the diameter of which is similar to that of the powders used in examples 1 and 2.

(28) The samples according to examples 1 to 4 are subsequently analyzed. The overall chemical composition of the grains, indicated as percentages by weight on the basis of the oxides, was determined by X-ray fluorescence. The results are combined in table 1 which follows:

(29) TABLE-US-00001 TABLE 1 Zirconia (ZrO.sub.2 + HfO.sub.2) Al.sub.2O.sub.3 MgO TiO.sub.2 SiO.sub.2 Fe.sub.2O.sub.3 CaO Example 1 95.8 0.2 3.3 0.1 0.1 0.1 0.2 (comparative) Example 2 68.9 21.0 9.7 0.1 0.1 0.1 0.1 (comparative) Example 3 50.1 34.4 14.8 0.1 0.2 0.2 0.2 (invention) Example 4 72.0 17.0 9.6 0.1 0.2 0.1 0.2 (invention)

(30) The qualitative analysis of the phases present in the grains according to examples 1 to 4 is subsequently determined by X-ray diffraction. The results are combined in table 2 which follows:

(31) TABLE-US-00002 TABLE 2 Phases detected Example 1 Tetragonal or cubic zirconia/monoclinic zirconia (comparative) Example 2 Tetragonal or cubic zirconia/spinel/monoclinic (comparative) zirconia Example 3 Cubic zirconia/spinel/monoclinic zirconia (invention) Example 4 Cubic zirconia/spinel (invention)

(32) The resistance with regard to temperature is evaluated by comparing the phases detected before and after having placed the samples of examples 1 to 4 at 1400 C. for 1 hour, that is to say at a temperature lower than the minimum sintering temperature described in the patent application CN101786889, section [0012].

(33) Such conditions also appear representative of the conditions undergone by a material formed from the different grains during its use as refractory component in metallurgy.

(34) As indicated above, the resistance with regard to temperature of said material is related to the degree of transformation of the zirconia phase from the cubic or tetragonal form toward the monoclinic form during a cycle comprising a rise and a fall in temperature, this transformation resulting in a known way in a microcracking of the constituent grains of the material and consequently in a deterioration in the macroscopic properties of the latter, as explained above.

(35) More specifically, for each example, the content of monoclinic zirconia is determined by X-ray diffraction before and after the test. The results are combined in table 3 which follows:

(36) TABLE-US-00003 TABLE 3 Monoclinic zirconia Before the test After the test Example 1 46% 70% (comparative) Example 2 29% 59% (comparative) Example 3 6% 34% (invention) Example 4 Not detected <0.5% (invention)

(37) The results given in table 3 show that the fused grains according to the invention exhibit a much improved stability with respect to the comparative examples. By reducing the changes in phases, in particular the change from cubic or tetragonal zirconia to monoclinic zirconia, associated dimensional variations are avoided and thus the risk of cracking of the refractory products comprising or consisting of these grains are avoided.

(38) The microstructure of the grains obtained according to example 4 was observed by electron microscopy.

(39) The compositions of the different phases constituting the grains can be obtained by wavelength spectrometry (Castaing EPMA microprobe). This measurement makes it possible to confirm the visual observations and to specify the compositions of the different phases and inclusions observed in the electron microscopy photographs of FIGS. 1 and 2.

(40) The photographs are given in FIGS. 1 and 2. The microstructure, very different from those observed to date on grains obtained by sintering in the solid state, exhibits two very different regions which are perfectly observable with the microscope: A matrix (1) exhibiting an alternating arrangement of zirconia phase and spinel phase, in the form of fibers or of short rods of very low thickness. This very fine microstructure is characteristic of a eutectic phase obtained from two crystalline types. Occlusions (2) of a zirconia phase, which occlusions are coated in said matrix of eutectic.

(41) Without it being possible for this explanation to be regarded as definitive, the stabilization of the zirconia observed might thus be related to this specific microstructure obtained by virtue of the overall chemical composition made of the different oxides as described above and in particular of the presence of the ZrO.sub.2-spinel eutectic phase surrounding occlusions of zirconia or of spinel, preferably of zirconia. By virtue of such a microstructure, it becomes possible to use materials comprising very high zirconia contents, very particularly for the manufacture of refractory materials.