EP3350138A1

EP3350138A1 - Melted magnesium aluminate grain rich in magnesium

Info

Publication number: EP3350138A1
Application number: EP16777726.7A
Authority: EP
Inventors: Stéphane RAFFY
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2015-09-14
Filing date: 2016-09-14
Publication date: 2018-07-25
Also published as: WO2017046517A1; JP2018530509A; US10494308B2; KR20180052717A; US20190039956A1; FR3040993A1

Abstract

The invention relates to a melted grain essentially consisting of a matrix of a magnesium and aluminium oxide of a spinel structure MgA12O4 and/or eutectic structure MgO-MgA12O4 and inclusions essentially consisting of magnesium oxide, said grain having the following overall chemical composition, in weight percentages expressed in the form of oxides: more than 5.0% and less than 19.9% of A12O3; A12O3 and MgO together representing more than 95.0% of the weight of said grain, said grain being characterised in that the cumulated content of CaO and ZrO2 is less than 4000 ppm, in weight. The invention also relates to a ceramic product or material produced by sintering said grains.

Description

MAGNESIUM RICH MAGNESIUM ALUMINATE FUSED GRAIN

The invention relates to grains for ceramic applications consisting essentially of oxides of Al and Mg elements in the form of magnesium - rich magnesium aluminate, often referred to as MMA for "magnesium rich magnesium aluminate" in the field. The invention also relates to a method of manufacturing such grains, and to materials, products or ceramic coatings made from said grains, often called MMA ceramics. Such materials find particular, but not only, their application in the manufacture of SOFC tube, or in the manufacture of supports for the separation of gases. Such materials can also be used for producing refractory parts for the manufacture or processing of metals or metal alloys. They can also be used as a coating for metal parts or in the case of contact between a ceramic part and a metal.

One of the essential features that has allowed the use of MMA ceramics in many technical fields is their coefficient of thermal expansion (CTE). In particular, previous studies have shown that the coefficient of thermal expansion of MMA ceramics is similar or very close to that of metals and that in addition it could be adapted, depending on the chemical composition of the ceramic material and its microstructure, to precisely match that of the metal with which it is in contact. For example, reference may be made to patent application WO 2004/030131 or US Pat. No. 6,723,442 B1 in this regard. Another characteristic of MMA ceramics for use at high temperature is their dimensional stability and in particular their resistance to creep. Creep means, in the sense of the present invention, the capacity of the material to deform under the effect of the stresses undergone when it is subjected to high temperatures.

However, MMA ceramics are difficult to form in the aqueous phase because of their sensitivity to water. Indeed, in the presence of water, the crystalline phases of the precursors usually used to obtain MMA ceramics are transformed and an Mg (OH) 2 brucite phase which appears at the expense of the MgO periclase phase. In the end, the presence of too much initial amount of this brucite phase causes a difficulty, or even an impossibility to shaping the product.

Thus, the present invention relates to molten grains of the MMA type which can be used for the manufacture of ceramic parts or coatings which can be more easily shaped in the aqueous phase.

According to another aspect of the present invention, said ceramic parts or coatings may have a substantially improved creep resistance, compared to the state of the art. Work carried out by the applicant company has indeed been able to highlight a link between the rate of certain impurities of the grains and the final performance of the creep resistance of the ceramic material obtained from them.

More specifically, the present invention relates to a melted grain (or a mixture of melted grains) consisting essentially of a matrix of a magnesium oxide and aluminum of spinel structure MgA ^ C ^ and / or of eutectic MgO-MgAl ₂ 0 ₄ and inclusions made up essentially of magnesium oxide, said inclusions being included in said matrix, said grain having the following overall chemical composition, in weight percentages expressed in the form of oxides:

- more than 5,0% and less than 19,9% of Al ₂ 0 ₃ ,

AI ₂ O ₃ and MgO together represent more than 95.0% of the weight of said grain,

said grain being characterized in that the cumulative rate of CaO and Zr0 ₂ is less than 4000 ppm, by weight.

More particularly, in a melted grain according to the invention, the magnesium oxide matrix phase of spinel structure MgA ^ C ^ and / or the eutectic MgO-MgAl ₂ 0 ₄ thus coat said inclusions constituted essentially of magnesium oxide, as it can be seen on the enclosed electron micrograph. It is considered that such a structure makes it possible to ensure the cohesion of the grain and its resistance to hydration.

The grain according to the invention may comprise, especially in the form of impurities, up to 5% of other oxides.

Preferably, the cumulative rate of CaO and ZrC ₂ is less than 3500 ppm, more preferably less than 3000 ppm, and very preferably less than 2500 ppm by weight. Cumulative rate means the sum of the CaO and ZrO _{2 levels} in the melted grains.

Preferably, the CaO level is less than 3500 ppm, more preferably less than 3000 ppm, even less than 2500 ppm, or even less than 2000 ppm, and very preferably less than 1500 ppm by weight.

Preferably, the level of ZrO ₂ is less than 3000 ppm, more preferably less than 2000 ppm, or even less than 1500 ppm, or even less than 1000 ppm, or even less than 500 ppm, and very preferably less than 200 ppm. weight. Preferably, the melted grain according to the invention does not comprise an Al ₂ O ₃ alumina structural phase.

Preferably, the cumulative rate of BaO and SrO is less than 3000 ppm, more preferably less than 2500 ppm, and most preferably less than 2000 ppm by weight. Cumulative rate means the sum of the BaO and SrO levels in the melted grains.

Preferably, the BaO content is less than 2500 ppm, more preferably less than 2000 ppm, or even less than 1500 ppm, or even less than 1000 ppm by weight.

Preferably, the SrO level is less than 2500 ppm, more preferably less than 2000 ppm, or even less than 1500 ppm, or even less than 1000 ppm by weight.

Preferably, the a20 level is less than 2500 ppm, more preferably less than 500 ppm, or even less than 300 ppm.

Preferably, the Fe ₂ O _{3 content} is less than 1000 ppm, more preferably less than 500 ppm, or even less than 750 ppm, or even less than 500 ppm by weight.

Preferably, the MnO 2 content is less than 500 ppm, more preferably less than 300 ppm, or even less than 200 ppm, or even less than 100 ppm by weight.

Preferably, the level of S 1 O 2 is less than 500 ppm, more preferably less than 200 ppm by weight.

According to one embodiment, the content of T 1 O 2 is less than 500 ppm by weight.

According to one embodiment, AI ₂ O ₃ represents more than 8.0% of the weight or even more than 10.0%, or even more than 12.0% of the weight of said grain. AI ₂ O ₃ may represent less than 19.5% of the weight of said grain, or even less than 19.0% or even less than 18.0% or even less than 17.0% of the weight of said grain.

Preferably, AI ₂ O ₃ and MgO together represent more than 96.0% of the weight of said grain. More preferably, AI ₂ O ₃ and MgO together represent more than 97.0% or more of 98.0% or even more than 99.0%, or even more than 99.2% and very preferably at least 99.4% of the weight of said grain.

Preferably, the matrix of the melted grain according to the invention consists of distinct zones of spinel structure and / or the eutectic MgO-MgAl ₂ 0 ₄ .

The melted grain according to the invention comprises fine inclusions consisting essentially of oxides of calcium and zirconium, the largest dimension of which is less than 2 micrometers, preferably less than 1 micrometer, on an electron microscope slide.

In particular, the cumulative sum of said oxides of calcium and zirconium may represent more than 80% of the mass of said inclusions and preferably more than 90% or even more than 95% of the mass of said inclusions, as for example measured by EPMA .

The present invention also relates to the ceramic material obtained by sintering melted grains as described above or by sintering a mixture comprising melted grains such as previously described, for example in the form of ceramic part or ceramic coating.

Such a material is in particular characterized in that it may comprise fine inclusions consisting essentially of oxides of calcium and zirconium, the number of which is less than 100 per 10,000 square micrometer, on an electron microscope slide.

The present invention relates in particular to a mixture of melted grains as described above.

According to a first embodiment, the melted grain mixture has a median particle size dso of the particles, as measured by laser particle size, of between 0.1 and 150 microns, in particular between 1 and 100 microns, and even between 2 and 70 microns. microns, more particularly between 0.1 and 50 microns. According to one In other embodiments, in some applications, the median particle size, as measured by laser particle size, is between 20 and 150 microns.

In still other applications, for example for the manufacture of refractory products, the median particle size of the particles may be up to 5 millimeters. It is for example between 1 micron and 5 millimeters, or even between 50 microns and 2 millimeters. It is for example between 500 microns and 5 millimeters.

The invention relates in particular to a mixture of melted grains as previously described.

The invention relates in particular to a mixture of melted grains having a median particle size of between 1 and 150 microns, especially between 1 and 50 microns, said grains comprising less than 50% by weight of magnesium hydroxide, after immersion of 5 grams of said mixture in 25 cm ³ of distilled water with stirring and at room temperature (25 ° C) for two hours.

For the purposes of the present invention, the following definitions and indications are given:

According to the habits in the field of ceramics, the chemical composition of the grains, in particular according to the main constitutive elements Al and Mg, is given in the present description, unless explicitly stated otherwise, with reference to the corresponding simple oxides AI 2O ₃ or MgO, even if said element is not present or only partially present in this form. It is the same for the impurities present in the grains, whose content is given by reference to the oxide of the corresponding element, even if said element is actually present in another form in said grains. Such a description is moreover consistent with the data usually provided by elemental chemical analysis according to the X-ray fluorescence devices usually used to determine the elemental composition of the materials.

By "eutectic MgO-MgAl ₂ 0 ₄ " is meant the MgO-spinel eutectic structure corresponding to the composition point, by weight, close to 55% of A 2 Os and 45% MgO and a temperature of about 2000 ° C. the phase diagram MgO-A1203 (invariant point of the phase diagram for which the liquid to solid reaction is complete). The term "melted grain" conventionally refers to a grain obtained by a manufacturing process comprising at least one melting step of an initial mixture of raw materials, a solidification step and a grinding step.

The term "melting" of an initial mixture of raw materials is a heat treatment at a temperature sufficiently high that all the constituents of the initial mixture are in the molten (liquid) state.

By "impurities" is meant in particular the inevitable constituents necessarily introduced with the raw materials. The impurities are introduced during the preliminary step of manufacturing the melted grains by the raw materials.

The main impurities depend, of course, on the raw materials used, most often commercial powders of MgO or Al ₂ O ₃ of purity greater than or equal to 95% by weight of the oxide. The main detectable impurities in the grain or the material according to the invention are most often and essentially oxides of calcium, iron, silicon, manganese, sodium, zirconium or even titanium.

The chemical analysis of the material obtained by sintering melted grains according to the invention is substantially identical to that of said melted grains. Thus, the various elements constituting the microstructure of the material obtained by sintering melted grains according to the invention are substantially identical to those of said melted grains.

The term "sintering", conventionally used in the field of ceramics, a consolidation by thermal treatment of a granular agglomerate, possibly with a partial or total melting of some of the constituents of said agglomerate, but without melting of at least 1 one of its constituents.

The sintering according to the invention is normally carried out in exclusively solid phase, and in particular all the constituents of the melted grains remain in solid phase during said sintering.

According to the invention, the sintering temperature of the melted grains is normally between 1200 ° C. and 1650 ° C.

A method of manufacturing the previously described grains comprises the following steps:

a) mixing raw materials, in particular magnesium oxide and aluminum oxide or a precursor thereof, to form the feedstock; b) melting of the feedstock until the molten liquid is obtained;

c) cooling said molten liquid so that the molten liquid is fully solidified, for example in less than 3 minutes;

d) grinding said solid mass so as to obtain a mixture of melted grains.

According to the invention, the raw materials are chosen in step a) so that the melted grains obtained in step d) are in accordance with the invention.

Of course, without departing from the scope of the invention, any other conventional or known method of manufacturing melted grains may also be implemented, provided that the composition of the feedstock allows to obtain grains having a composition in accordance with that of the grains of the invention.

In step b), an electric arc furnace is preferably used, but all known furnaces are conceivable, such as an induction furnace or a plasma furnace, provided that they allow the initial charge to be completely melted. The melting is preferably carried out under neutral conditions, for example under argon, or oxidizing, preferably at atmospheric pressure. As indicated above, step b) is carried out at a temperature allowing complete melting of the feedstock. Such a temperature is higher than that of the eutectic MgO-MgAl ₂ 0 ₄ , that is to say at a temperature above 2000 ° C, more preferably above 2050 ° C.

In step c), the cooling can be rapid, that is to say that the molten liquid is fully solidified in less than 3 minutes. Preferably it results from casting in CS molds as described in US Pat. No. 3,993,119 or a quenching or by a blowing technique.

In step d), the solid mass is milled, according to conventional techniques, to obtain the grain size suitable for the intended application. For example, the grinding can be continued until obtaining grains having a median size dso particles, as measured by laser particle size, for example between 2 and 50 microns, or even between 0.1 and 50 microns in some applications or between 20 and 150 microns in other applications (eg thermal spraying) or even up to 5 millimeters for applications such as the manufacture of refractory materials. The invention and its advantages will be better understood on reading the nonlimiting examples which follow. In the examples, unless otherwise indicated, all percentages as well as ppm (parts per million) are given by weight.

Example 1

In this comparative example, a mixture is prepared from the following commercial raw materials:

a magnesium oxide powder having the following chemical analysis (in weight percentages): MgO>99%; CaO <0.1%; Na ₂ 0 <0.1%; Si0 ₂

<0.05%; Zr0 ₂ ≤ 0.02%; Fe ₂ O ₃ ≤ 0.02%;

an aluminum oxide powder with Al ₂ 0 ₃ ≥ 99% by weight) having the following impurities (in weight percentage): Na ₂ O: 0.23%, CaO <0.02%, MgO

<0.05%, Fe ₂ O ₃ <0.02%, SiO ₂ <0.05%, ZrO ₂ <0.02%, TiO ₂ <0.02%.

The mixture consists solely of these two commercial powders mixed in a mass ratio MgO / Al ₂ O ₃ of 84.3 / 15.7; then coiled until a median particle size dso of the particles, as measured by laser granulometry, of the order of 3.4 micrometers.

The elemental analysis by X-ray fluorescence of the mixture thus obtained makes it possible to determine, with a relative uncertainty of the order of 1%, the concentrations of oxides. The blend has an aluminum oxide content of 15.7 weight percent and a magnesium oxide content of 84.2 weight percent. The impurities detected are calcium (700 ppm of CaO equivalent) and sodium (400 ppm of Na ₂ O equivalent); the other species are below the detection thresholds of the measuring device, the iron (<200 ppm Fe ₂ O ₃ equivalent), silicon (<500 ppm S1O ₂ equivalent), zirconium (<500 ppm ZrC equivalent> 2) and titanium (<120 ppm TiO 2 equivalent) ) · The composition of the mixture thus obtained is determined according to the following protocol:

Phase analysis: The analysis of the sample by X - ray diffraction makes it possible to identify the different crystalline phases of the sample. It is carried out using the EVA® software and the ICDD PDF-2 database (2005 version). The proportions of the different phases are then determined by the Rietveld method using the software HighScore Plus 4.0 (PANalytical BV). The .cif files of the "Inorganic Crystal Structure Database" database: ICSD # 9863 (for periclase MgO), # 203212 (for brucite Mg (OH) ₂ ), # 51687 (for AI ₂ O3 corundum) , and ## EQU1 ## (for the spinel MgA ^ C ^) are used as the starting point of the refinement. The Bragg peaks are modeled with "Pseudo-Voigt" functions.

The phase analysis carried out on the mixture obtained at the end of the co-grinding shows that it consists essentially of a MgO periclase phase and an Al ₂ O ₃ corundum phase.

The resistance to hydration of the mixture prepared as previously described is measured according to the following protocol:

A preliminary phase analysis is performed on the sample. The resistance to hydration is measured by placing 5 grams of the mixture in 25 cm ³ of distilled water with stirring and at room temperature for two hours. The sample is then dried for 24 hours at 110 ° C. before a new phase analysis and a weighing. The comparison of the two phase analyzes according to the protocol described above and weighing before and after the hydration test makes it possible to determine the sensitivity of the sample to the hydration according to the following two criteria:

- the more the brucite phase appears, the more sensitive the sample,

the more the mass is greater than 5 grams, the more the sample is hydrated and therefore sensitive to hydration.

The results obtained for the comparative sample according to this example 1 are reported in Table 1 below.

Example 2 (according to the invention)

In this example according to the invention, melted grains are prepared by melting from the commercial raw materials used in Example 1, mixed in a mass ratio MgO / Al ₂ O ₃ of the order of 85/15.

The powder mixture is melted in an arc furnace at a temperature of about 2100 to 2300 ° C. The molten liquid is solidified and cooled. The molten product is then milled until obtaining a powder of melted grains whose median particle size dso, as measured by laser granulometry, is of the order of 2.9 microns and whose particle size distribution is similar to that of Example 1.

Elemental analysis by X-ray fluorescence of the melted grains thus obtained makes it possible to determine, as in Example 1, the concentrations of elemental oxides.

The grains have an aluminum oxide content of 15.7 weight percent and a magnesium oxide content of 84.2 weight percent. The impurities detected are the calcium (1200 ppm CaO equivalent); the other species are below the detection limits of the measuring apparatus: iron (<200 ppm Fe ₂ equivalent ₃ ), silicon (<500 ppm SiO ₂ equivalent), zirconium (<500 ppm Zr0 equivalent ₂ ) and titanium (<100 ppm equivalent TiO ₂ ).

The phase analysis carried out on the mixture of grains according to the previously described protocol shows that these consist essentially of a MgO periclase phase and a MgAl ₂ O ₄ spinel phase.

In the attached figure, an electron micrograph is presented on a molten grain obtained according to the invention: a very particular structure is observed, in which zones 1 consisting of the spinel phase MgAl ₂ O 4 and / or the The eutectic MgO-MgAl ₂ 04 comprises, according to a matrix structure, grains 2 consisting essentially of the MgO periclase phase, as can be directly demonstrated by elemental analysis by a Castaing microprobe (also known as electron probe microanalyser, EPMA). The darkest areas 3 on the plate correspond to the porous zones of the structure.

The resistance to hydration of the melted grains thus prepared is measured according to the same protocols as previously described in relation to Example 1.

The results obtained are reported in Table 1 below.

Table 1 The results reported in Table 1 above show the advantages of using molten grains according to the invention in an aqueous phase shaping process because only a limited amount of brucite is detected. However, the observations made by the applicant company have shown that the presence of an excessive proportion of this phase makes the shaping in the aqueous route very difficult, if not impossible, as described below:

The mixture of co-milled powders of Example 1 and the powder of melted grains of Example 2 were suspended under conventional aqueous ceramic shaping conditions: 60% by weight of ceramic powder were 40% by weight of demineralized water were mixed with magnetic stirring. For 100 gram of ceramic powder, 1 gram of DOLAPIX® dispersant is added.

In the suspension using as a ceramic powder the mixture of co-milled powders of Example 1, a gel is then formed. No formatting could be possible from this suspension.

After 45 minutes, the suspension using the powder of melted grains according to Example 2 did not form any visible gel. A shaping of this suspension could be carried out without difficulties.

Claims

Melted grain consisting essentially of a matrix of magnesium oxide and aluminum of spinel structure MgA ^ C ^ and / or eutectic MgO-MgAl ₂ 0 ₄ , and inclusions consisting essentially of magnesium oxide said grain exhibiting the following overall chemical composition, in percentages by weight expressed in the form of oxides:

- more than 5,0% and less than 19,9% of Al ₂ 0 ₃ ,

said grain being characterized in that the cumulative rate of CaO and ZrO 2 is less than 4000 ppm, by weight.

Melted grain according to claim 1 wherein AI ₂ O ₃ is greater than 8.0% by weight.

3. Melted grain according to one of the preceding claims, wherein the cumulative rate of CaO and ZrC> 2 is less than 3000 ppm.

Melted grain according to one of the preceding claims, wherein the cumulative rate of CaO and ZrO ₂ is less than 2500 ppm.

Melted grain according to one of the preceding claims, characterized in that it does not comprise alumina phase AI ₂ O ₃ .

Melted grain according to one of the preceding claims, in which the impurities are essentially CaO, ZrO ₂ , Fe ₂ O ₃ , SiO ₂ , Na ₂ O, MnO ₂ .

7. Melted grain according to one of the preceding claims, comprising less than 2000 ppm of CaO.

8. Melted grain according to one of the preceding claims, comprising less than 200 ppm Zr0 ₂ -

9. Melted grain according to one of the preceding claims, wherein AI ₂ O ₃ and MgO together represent more than 99.0% of the weight of said grain.

10. Melted grain according to one of the preceding claims, wherein the matrix consists of distinct zones of spinel structure and the eutectic MgO-MgAl ₂ 0 ₄ .

11. Melted grain according to one of the preceding claims, comprising fine inclusions consisting essentially of oxides of calcium and zirconium, the largest dimension is less than 2 microns, preferably less than 1 micrometer.

Mixture of melted grains according to one of the preceding claims.

13. Ceramic material obtained by sintering melted grains according to one of the preceding claims. 14. Ceramic material according to the preceding claim, comprising inclusions consisting essentially of oxides of calcium and zirconium, the number of which is less than 100 per 10,000 square micrometer, on an electron microscope slide.

15. Process for producing grain according to one of claims 1 to 11, comprising the following steps:

a) mixing raw materials to form the feedstock,

b) melting of the feedstock to obtain the molten liquid,

c) cooling said molten liquid so that the molten liquid is fully solidified, d) grinding said solid mass so as to obtain a mixture of said molten grains.